Add //flutter/fml/math.h with common math constants. (flutter#30425)

This came up as I was [debugging failures on Windows](https://ci.chromium.org/p/flutter/builders/try/Windows%20Host%20Engine/30618?) due to use of the
non-portable constants.

Fixes flutter/flutter#82216

ci/licenses_golden/licenses_flutter

@@ -176,6 +176,8 @@ FILE: ../../../flutter/fml/make_copyable.h
 FILE: ../../../flutter/fml/mapping.cc
 FILE: ../../../flutter/fml/mapping.h
 FILE: ../../../flutter/fml/mapping_unittests.cc
+FILE: ../../../flutter/fml/math.h
+FILE: ../../../flutter/fml/math_unittests.cc
 FILE: ../../../flutter/fml/memory/ref_counted.h
 FILE: ../../../flutter/fml/memory/ref_counted_internal.h
 FILE: ../../../flutter/fml/memory/ref_counted_unittest.cc

fml/BUILD.gn

@@ -37,6 +37,7 @@ source_set("fml") {
     "make_copyable.h",
     "mapping.cc",
     "mapping.h",
+    "math.h",
     "memory/ref_counted.h",
     "memory/ref_counted_internal.h",
     "memory/ref_ptr.h",
@@ -272,6 +273,7 @@ if (enable_unittests) {
       "hex_codec_unittest.cc",
       "logging_unittests.cc",
       "mapping_unittests.cc",
+      "math_unittests.cc",
       "memory/ref_counted_unittest.cc",
       "memory/task_runner_checker_unittest.cc",
       "memory/weak_ptr_unittest.cc",

fml/math.h

@@ -0,0 +1,48 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+namespace flutter {
+namespace math {
+
+// e
+constexpr float kE = 2.7182818284590452354;
+
+// log_2 e
+constexpr float kLog2_E = 1.4426950408889634074;
+
+// log_10 e
+constexpr float kLog10_E = 0.43429448190325182765;
+
+// log_e 2
+constexpr float klogE_2 = 0.69314718055994530942;
+
+// log_e 10
+constexpr float klogE_10 = 2.30258509299404568402;
+
+// pi
+constexpr float kPi = 3.14159265358979323846;
+
+// pi/2
+constexpr float kPiOver2 = 1.57079632679489661923;
+
+// pi/4
+constexpr float kPiOver4 = 0.78539816339744830962;
+
+// 1/pi
+constexpr float k1OverPi = 0.31830988618379067154;
+
+// 2/pi
+constexpr float k2OverPi = 0.63661977236758134308;
+
+// 2/sqrt(pi)
+constexpr float k2OverSqrtPi = 1.12837916709551257390;
+
+// sqrt(2)
+constexpr float kSqrt2 = 1.41421356237309504880;
+
+// 1/sqrt(2)
+constexpr float k1OverSqrt2 = 0.70710678118654752440;
+
+}  // namespace math
+}  // namespace flutter

fml/math_unittests.cc

@@ -0,0 +1,27 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include <cmath>
+
+#include "flutter/fml/math.h"
+#include "gtest/gtest.h"
+
+namespace flutter {
+namespace testing {
+
+TEST(MathTest, Constants) {
+  // Don't use the constants in cmath as those aren't portable.
+  EXPECT_FLOAT_EQ(std::log2(math::kE), math::kLog2_E);
+  EXPECT_FLOAT_EQ(std::log10(math::kE), math::kLog10_E);
+  EXPECT_FLOAT_EQ(std::log(2.0f), math::klogE_2);
+  EXPECT_FLOAT_EQ(math::kPi / 2.0f, math::kPiOver2);
+  EXPECT_FLOAT_EQ(math::kPi / 4.0f, math::kPiOver4);
+  EXPECT_FLOAT_EQ(1.0f / math::kPi, math::k1OverPi);
+  EXPECT_FLOAT_EQ(2.0f / math::kPi, math::k2OverPi);
+  EXPECT_FLOAT_EQ(std::sqrt(2.0f), math::kSqrt2);
+  EXPECT_FLOAT_EQ(1.0f / std::sqrt(2.0f), math::k1OverSqrt2);
+}
+
+}  // namespace testing
+}  // namespace flutter