Preserve alpha channel of 1.0 when emulating RGB back buffer.

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

Index: third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
diff --git a/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp b/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
index 7a26d0f0102499c20e5b3afe6f6e882fb03cdeeb..c41262a7c84d208adb2f66613bf496d4255bb80c 100644
--- a/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
+++ b/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
@@ -232,6 +232,10 @@ bool DrawingBuffer::DefaultBufferRequiresAlphaChannelToBePreserved() {
   return !want_alpha_channel_ && rgb_emulation;
 }
 
+void DrawingBuffer::MarkBlitFramebufferUsedThisFrame() {
+  blit_framebuffer_used_this_frame_ = true;
+}
+
 std::unique_ptr<cc::SharedBitmap> DrawingBuffer::CreateOrRecycleBitmap() {
   auto it = std::remove_if(
       recycled_bitmaps_.begin(), recycled_bitmaps_.end(),
@@ -413,6 +417,7 @@ bool DrawingBuffer::FinishPrepareTextureMailboxGpu(
   front_color_buffer_ = color_buffer_for_mailbox;
 
   contents_changed_ = false;
+  blit_framebuffer_used_this_frame_ = false;
   SetBufferClearNeeded(true);
   return true;
 }
@@ -1019,6 +1024,27 @@ void DrawingBuffer::ResolveMultisampleFramebufferInternal() {
 void DrawingBuffer::ResolveIfNeeded() {
   if (anti_aliasing_mode_ != kNone)
     ResolveMultisampleFramebufferInternal();
+
+  // If the user performed a manual BlitFramebuffer to the default
+  // back buffer, they may have populated the alpha channel with
+  // values not equal to 1.0. Must undo this.
+  if (DefaultBufferRequiresAlphaChannelToBePreserved() &&
+      blit_framebuffer_used_this_frame_) {
+    // Avoid repeating this work if it's already been done.
+    if (anti_aliasing_mode_ != kNone &&
+        ContextProvider()
+            ->GetCapabilities()
+            .disable_multisampling_color_mask_usage) {
+      // Do nothing.
+    } else {
+      DCHECK(state_restorer_);
+      state_restorer_->SetClearStateDirty();
+      gl_->ClearColor(0, 0, 0, 1);
+      gl_->ColorMask(false, false, false, true);
+      gl_->Clear(GL_COLOR_BUFFER_BIT);
+    }
+  }
+
   contents_change_resolved_ = true;
 }