Switch WebGL's back buffer back to 4x MSAA from 8x MSAA.

8x MSAA caused performance regressions across a large range of GPUs
and operating systems. WebGL 2.0 applications have full control of
multisampling, and can use an 8x MSAA framebuffer if desired. Restore
4x MSAA for WebGL's back buffer.

Leave the associated driver bug workaround in place on the various
platforms, as it's passed into Skia and used there.

Bug: 1063437
Change-Id: I3bfa3905bc02794c7a503164f802b193bd9a66b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2524121
Commit-Queue: Shrek Shao <shrekshao@google.com>
Commit-Queue: James Darpinian <jdarpinian@chromium.org>
Auto-Submit: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Shrek Shao <shrekshao@google.com>
Reviewed-by: James Darpinian <jdarpinian@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825620}

content/renderer/webgraphicscontext3d_provider_impl.cc

@@ -91,8 +91,6 @@ WebGraphicsContext3DProviderImpl::GetWebglPreferences() const {
 
     if (gpu_feature_info.IsWorkaroundEnabled(MAX_MSAA_SAMPLE_COUNT_2))
       prefs.msaa_sample_count = 2;
-    else if (gpu_feature_info.IsWorkaroundEnabled(MAX_MSAA_SAMPLE_COUNT_4))
-      prefs.msaa_sample_count = 4;
 
     if (command_line->HasSwitch(switches::kWebglMSAASampleCount)) {
       std::string sample_count =

gpu/config/gpu_driver_bug_list.json

@@ -3658,6 +3658,6 @@
       "features": [
         "max_msaa_sample_count_4"
       ]
-    } 
+    }
   ]
 }

third_party/blink/public/platform/web_graphics_context_3d_provider.h

@@ -77,7 +77,7 @@ enum AntialiasingMode {
 
 struct WebglPreferences {
   AntialiasingMode anti_aliasing_mode = kAntialiasingModeUnspecified;
-  uint32_t msaa_sample_count = 8;
+  uint32_t msaa_sample_count = 4;
   uint32_t eqaa_storage_sample_count = 4;
   // WebGL-specific numeric limits.
   uint32_t max_active_webgl_contexts = 0;