Fix BlitFramebuffer RGB emulation on macOS.

The fix for Issue 699566 regressed blits between user framebuffers on
macOS when alpha:false and antialias:false. Only apply the workaround
when blitting to the default WebGL back buffer.

Tested by new conformance test
conformance2/rendering/blitframebuffer-r11f-g11f-b10f.html from
https://github.com/KhronosGroup/WebGL/pull/2469 .

BUG=751206
TBR=zmo@chromium.org
NOTRY=true

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
Change-Id: I6025bcd562675318d6cd9d3fcc50aa90fa83642b
Reviewed-on: https://chromium-review.googlesource.com/596679Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491300}

third_party/WebKit/Source/modules/webgl/WebGL2RenderingContextBase.cpp

@@ -404,7 +404,7 @@ void WebGL2RenderingContextBase::blitFramebuffer(GLint src_x0,
     return;
 
   DrawingBuffer::ScopedRGBEmulationForBlitFramebuffer emulation(
-      GetDrawingBuffer());
+      GetDrawingBuffer(), !!GetFramebufferBinding(GL_DRAW_FRAMEBUFFER));
   ContextGL()->BlitFramebufferCHROMIUM(src_x0, src_y0, src_x1, src_y1, dst_x0,
                                        dst_y0, dst_x1, dst_y1, mask, filter);
 }

third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp

@@ -580,9 +580,11 @@ DrawingBuffer::CreateOrRecycleColorBuffer() {
 }
 
 DrawingBuffer::ScopedRGBEmulationForBlitFramebuffer::
-    ScopedRGBEmulationForBlitFramebuffer(DrawingBuffer* drawing_buffer)
+    ScopedRGBEmulationForBlitFramebuffer(DrawingBuffer* drawing_buffer,
+                                         bool is_user_draw_framebuffer_bound)
     : drawing_buffer_(drawing_buffer) {
-  doing_work_ = drawing_buffer->SetupRGBEmulationForBlitFramebuffer();
+  doing_work_ = drawing_buffer->SetupRGBEmulationForBlitFramebuffer(
+      is_user_draw_framebuffer_bound);
 }
 
 DrawingBuffer::ScopedRGBEmulationForBlitFramebuffer::
@@ -1295,14 +1297,21 @@ GLenum DrawingBuffer::GetMultisampledRenderbufferFormat() {
   return GL_RGB8_OES;
 }
 
-bool DrawingBuffer::SetupRGBEmulationForBlitFramebuffer() {
+bool DrawingBuffer::SetupRGBEmulationForBlitFramebuffer(
+    bool is_user_draw_framebuffer_bound) {
   // We only need to do this work if:
+  //  - We are blitting to the default framebuffer
   //  - The user has selected alpha:false and antialias:false
   //  - We are using CHROMIUM_image with RGB emulation
   // macOS is the only platform on which this is necessary.
 
-  if (want_alpha_channel_ || anti_aliasing_mode_ != kNone)
+  if (is_user_draw_framebuffer_bound) {
     return false;
+  }
+
+  if (want_alpha_channel_ || anti_aliasing_mode_ != kNone) {
+    return false;
+  }
 
   if (!(ShouldUseChromiumImage() &&
         ContextProvider()->GetCapabilities().chromium_image_rgb_emulation))

third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.h

@@ -236,7 +236,8 @@ class PLATFORM_EXPORT DrawingBuffer
   // store and RGB emulation is in use (basically, macOS only).
   class PLATFORM_EXPORT ScopedRGBEmulationForBlitFramebuffer {
    public:
-    ScopedRGBEmulationForBlitFramebuffer(DrawingBuffer*);
+    ScopedRGBEmulationForBlitFramebuffer(DrawingBuffer*,
+                                         bool is_user_draw_framebuffer_bound);
     ~ScopedRGBEmulationForBlitFramebuffer();
 
    private:
@@ -457,7 +458,7 @@ class PLATFORM_EXPORT DrawingBuffer
 
   // Helpers to ensure correct behavior of BlitFramebuffer when using
   // an emulated RGB CHROMIUM_image back buffer.
-  bool SetupRGBEmulationForBlitFramebuffer();
+  bool SetupRGBEmulationForBlitFramebuffer(bool is_user_draw_framebuffer_bound);
   void CleanupRGBEmulationForBlitFramebuffer();
 
   // Weak, reset by beginDestruction.