Support checking for GL_CHROMIUM_color_buffer_float_rgb[a] for float renderability.

ANGLE now exposes this extension to indicate that floating point textures
are renderable so it is the preferred way to determine if these extensions
should be exposed.

BUG=angleproject:1958
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2795123003
Cr-Commit-Position: refs/heads/master@{#469325}

gpu/command_buffer/service/feature_info.cc

@@ -1414,7 +1414,7 @@ void FeatureInfo::InitializeFloatAndHalfFloatFeatures(
         enable_texture_float_linear = true;
       }
 
-      if (enable_ext_color_buffer_float || gl_version_info_->is_angle) {
+      if (enable_ext_color_buffer_float) {
         may_enable_chromium_color_buffer_float = true;
       }
     }
@@ -1451,7 +1451,25 @@ void FeatureInfo::InitializeFloatAndHalfFloatFeatures(
     }
   }
 
-  if (may_enable_chromium_color_buffer_float) {
+  bool had_native_chromium_color_buffer_float_ext = false;
+  if (extensions.Contains("GL_CHROMIUM_color_buffer_float_rgb")) {
+    had_native_chromium_color_buffer_float_ext = true;
+    feature_flags_.chromium_color_buffer_float_rgb = true;
+    if (!disallowed_features_.chromium_color_buffer_float_rgb) {
+      EnableCHROMIUMColorBufferFloatRGB();
+    }
+  }
+
+  if (extensions.Contains("GL_CHROMIUM_color_buffer_float_rgba")) {
+    had_native_chromium_color_buffer_float_ext = true;
+    feature_flags_.chromium_color_buffer_float_rgba = true;
+    if (!disallowed_features_.chromium_color_buffer_float_rgba) {
+      EnableCHROMIUMColorBufferFloatRGBA();
+    }
+  }
+
+  if (may_enable_chromium_color_buffer_float &&
+      !had_native_chromium_color_buffer_float_ext) {
     static_assert(GL_RGBA32F_ARB == GL_RGBA32F &&
                       GL_RGBA32F_EXT == GL_RGBA32F &&
                       GL_RGB32F_ARB == GL_RGB32F && GL_RGB32F_EXT == GL_RGB32F,