Ensure alpha channel in backbuffer is correct with DirectComposition.

gpu/command_buffer/service/gles2_cmd_decoder.cc

Index: gpu/command_buffer/service/gles2_cmd_decoder.cc
diff --git a/gpu/command_buffer/service/gles2_cmd_decoder.cc b/gpu/command_buffer/service/gles2_cmd_decoder.cc
index 026e2a989eaae7f371e8d67e8713776eac1af494..94a544c34b69ef4e3ce6a490c8fe3b57f01797a1 100644
--- a/gpu/command_buffer/service/gles2_cmd_decoder.cc
+++ b/gpu/command_buffer/service/gles2_cmd_decoder.cc
@@ -1979,6 +1979,8 @@ class GLES2DecoderImpl : public GLES2Decoder, public ErrorStateClient {
   // Backbuffer attachments that are currently undefined.
   uint32_t backbuffer_needs_clear_bits_;
 
+  uint64_t swaps_since_resize_;
+
   // The current decoder error communicates the decoder error through command
   // processing functions that do not return the error value. Should be set only
   // if not returning an error.
@@ -2524,6 +2526,7 @@ GLES2DecoderImpl::GLES2DecoderImpl(ContextGroup* group)
       back_buffer_draw_buffer_(GL_BACK),
       surfaceless_(false),
       backbuffer_needs_clear_bits_(0),
+      swaps_since_resize_(0),
       current_decoder_error_(error::kNoError),
       validators_(group_->feature_info()->validators()),
       feature_info_(group_->feature_info()),
@@ -3642,8 +3645,7 @@ bool GLES2DecoderImpl::CheckFramebufferValid(
     if (surfaceless_)
       return false;
     if (backbuffer_needs_clear_bits_) {
-      glClearColor(0, 0, 0, (GLES2Util::GetChannelsForFormat(
-          offscreen_target_color_format_) & 0x0008) != 0 ? 0 : 1.f);
+      glClearColor(0, 0, 0, BackBufferHasAlpha() ? 0 : 1.f);
       state_.SetDeviceColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
       glClearStencil(0);
       state_.SetDeviceStencilMaskSeparate(GL_FRONT, kDefaultStencilMask);
@@ -4338,8 +4340,13 @@ error::Error GLES2DecoderImpl::HandleResizeCHROMIUM(
                  << "current after resize callback.";
       return error::kLostContext;
     }
+    if (surface_->BuffersFlipped()) {
+      backbuffer_needs_clear_bits_ |= GL_COLOR_BUFFER_BIT;
+    }
   }
 
+  swaps_since_resize_ = 0;
+
   return error::kNoError;
 }
 
@@ -12193,6 +12200,12 @@ void GLES2DecoderImpl::FinishSwapBuffers(gfx::SwapResult result) {
       group_->LoseContexts(error::kUnknown);
     }
   }
+  ++swaps_since_resize_;
+  if (swaps_since_resize_ == 1 && surface_->BuffersFlipped()) {
+    // The second buffer after a resize is new and needs to be cleared to
+    // known values.
+    backbuffer_needs_clear_bits_ |= GL_COLOR_BUFFER_BIT;
+  }
 }
 
 void GLES2DecoderImpl::DoCommitOverlayPlanes() {