Add //base dependency to rtc_base:rtc_event when building with chromium.

Should fix flakes of the form:

In file included from ../../third_party/webrtc/rtc_base/platform_thread.cc:11:
In file included from ../../third_party/webrtc\rtc_base/platform_thread.h:17:
In file included from ../../third_party/webrtc_overrides\rtc_base/event.h:9:
In file included from ../..\base/synchronization/waitable_event.h:15:
In file included from ../..\base/win/scoped_handle.h:12:
../..\base/location.h(14,10):  fatal error: 'base/debug/debugging_buildflags.h' file not found
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Bug: chromium:799759
Change-Id: I24a329e944c3ec8324057d882f96648c10198022
Reviewed-on: https://webrtc-review.googlesource.com/64388
Reviewed-by: Oleh Prypin <[email protected]>
Reviewed-by: Patrik Höglund <[email protected]>
Commit-Queue: Oleh Prypin <[email protected]>
Cr-Commit-Position: refs/heads/master@{#22608}

rtc_base/BUILD.gn

@@ -180,7 +180,14 @@ rtc_source_set("platform_thread") {
 }
 
 rtc_source_set("rtc_event") {
+  deps = [
+    ":checks",
+    ":macromagic",
+  ]
+
   if (build_with_chromium) {
+    # Dependency on chromium's waitable_event (in //base).
+    deps += [ "//base:base" ]
     sources = [
       "../../webrtc_overrides/rtc_base/event.cc",
       "../../webrtc_overrides/rtc_base/event.h",
@@ -191,11 +198,6 @@ rtc_source_set("rtc_event") {
       "event.h",
     ]
   }
-
-  deps = [
-    ":checks",
-    ":macromagic",
-  ]
 }
 
 rtc_source_set("logging") {