[Impeller] Added a doc page to point out important impeller benchmarks (

flutter#44333)

My main goal here was to get a more clear picture of how the Vulkan implementation is performing in relation to the OpenGLES implementation.  There is a lot of conventional wisdom about what benchmarks to look at that isn't documented anywhere.  As we come to rely on benchmarks daily this should be updated.

issue: flutter/flutter#131784
issue: flutter/flutter#131782

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

impeller/README.md

@@ -192,3 +192,4 @@ To your `AndroidManifest.xml` file, add under the `<application>` tag:
 * [Guidance for writing efficient shaders](docs/shader_optimization.md)
 * [How color blending works in Impeller](docs/blending.md)
 * [Enabling Vulkan Validation Layers on Android](docs/android_validation_layers.md)
+* [Important Benchmarks](docs/benchmarks.md)

impeller/docs/benchmarks.md

@@ -0,0 +1,24 @@
+# Impeller Benchmarks
+
+Here are some noteworthy benchmarks related to Impeller performance:
+
+- **New Gallery** - Runs through the Flutter Gallery with a driver test.
+  - Samsung S10
+    - Vulkan: [skiaperf](https://flutter-flutter-perf.skia.org/e/?keys=X8f96868d3a9eeb120bec1f458c577c30)
+    - OpenGLES: [skiaperf](https://flutter-flutter-perf.skia.org/e/?keys=Xeb13bfef4ef2947f899646422bbad8c6)
+    - Vulkan vs OpenGLES - average rasterizer time: [skiaperf](https://flutter-flutter-perf.skia.org/e/?keys=Xdfca283b38a86fc09129141792cf5a4b)
+    - Skia vs Vulkan Impeller - 90th percentile frame rasterizer time: [skiaperf](https://flutter-flutter-perf.skia.org/e/?keys=X2cacf305c9d4b1b5fc43f81368803a9b)
+  - Moto G4 (OpenGLES): [skiaperf](https://flutter-flutter-perf.skia.org/e/?keys=Xaeae5aa39c9028be43e8a9ad40540bd8)
+  - iPhone 11
+    - Metal: [skiaperf](https://flutter-flutter-perf.skia.org/e/?keys=Xc30b4774a54a03180fa93bf6641c5469)
+    - Skia vs Metal Impeller - 90th percentile frame rasterizer time: [skiaperf](https://flutter-flutter-perf.skia.org/e/?keys=X836c18b955eb83a9102a4391672f37e0)
+
+- **Animated Blur Backdrop Filter** - A driver test that scrolls to a screen and
+  animates a Blur Backdrop filter to get progressively blurrier.  This covers a
+  gap in the "New Gallery" tests we've seen in places like Wonderous.
+  - Samsung S10
+    - Vulkan: [skiaperf](https://flutter-flutter-perf.skia.org/e/?keys=X71aab43432178775be19fe133cdb5528)
+    - OpenGLES: [skiaperf](https://flutter-flutter-perf.skia.org/e/?keys=X8024e2cd402a6afcefdb18aaabc9533a)
+    - Vulkan vs OpenGLES - Average rasterizer time: [skiaperf](https://flutter-flutter-perf.skia.org/e/?keys=Xb1c6d1bb2e43c633bc3e1aa896cf5b08)
+  - Moto G4 (OpenGLES): [skiaperf](https://flutter-flutter-perf.skia.org/e/?keys=X78023772ea9e94c81f37456a7fa7bf46)
+  - iPhone 11 (Metal): [skiaperf](https://flutter-flutter-perf.skia.org/e/?keys=X2f7504aba3db6aeff08cc896081ace55)