Ensure alpha channel in backbuffer is correct with DirectComposition.

gpu/command_buffer/service/gles2_cmd_decoder_unittest_framebuffers.cc

Index: gpu/command_buffer/service/gles2_cmd_decoder_unittest_framebuffers.cc
diff --git a/gpu/command_buffer/service/gles2_cmd_decoder_unittest_framebuffers.cc b/gpu/command_buffer/service/gles2_cmd_decoder_unittest_framebuffers.cc
index 3383e26971b8c93f7eb902e3de05cf15a60e05de..1c443a7dec7b59ec781e2fe006e7ab9672570b00 100644
--- a/gpu/command_buffer/service/gles2_cmd_decoder_unittest_framebuffers.cc
+++ b/gpu/command_buffer/service/gles2_cmd_decoder_unittest_framebuffers.cc
@@ -2767,6 +2767,43 @@ TEST_P(GLES2DecoderManualInitTest, InvalidateFramebufferBinding) {
       gfx::MockGLInterface::GetGLProcAddress("glDiscardFramebufferEXT"));
 }
 
+TEST_P(GLES2DecoderManualInitTest, ClearBackbufferBitsOnFlipSwap) {
+  InitState init;
+  init.gl_version = "opengl es 2.0";
+  InitDecoder(init);
+  surface_->set_buffers_flipped(true);
+
+  EXPECT_EQ(0u, GetAndClearBackbufferClearBitsForTest());
+
+  SwapBuffers& cmd = *GetImmediateAs<SwapBuffers>();
+  cmd.Init();
+  EXPECT_EQ(error::kNoError, ExecuteCmd(cmd));
+  EXPECT_EQ(GL_NO_ERROR, GetGLError());
+  EXPECT_EQ(static_cast<uint32_t>(GL_COLOR_BUFFER_BIT),
+            GetAndClearBackbufferClearBitsForTest());
+
+  EXPECT_EQ(error::kNoError, ExecuteCmd(cmd));
+  EXPECT_EQ(GL_NO_ERROR, GetGLError());
+  EXPECT_EQ(0u, GetAndClearBackbufferClearBitsForTest());
+
+  ResizeCHROMIUM& resize_cmd = *GetImmediateAs<ResizeCHROMIUM>();
+  resize_cmd.Init(1, 1, 1.0f, GL_TRUE);
+  EXPECT_EQ(error::kNoError, ExecuteCmd(resize_cmd));
+  EXPECT_EQ(GL_NO_ERROR, GetGLError());
+  EXPECT_EQ(static_cast<uint32_t>(GL_COLOR_BUFFER_BIT),
+            GetAndClearBackbufferClearBitsForTest());
+
+  cmd.Init();
+  EXPECT_EQ(error::kNoError, ExecuteCmd(cmd));
+  EXPECT_EQ(GL_NO_ERROR, GetGLError());
+  EXPECT_EQ(static_cast<uint32_t>(GL_COLOR_BUFFER_BIT),
+            GetAndClearBackbufferClearBitsForTest());
+
+  EXPECT_EQ(error::kNoError, ExecuteCmd(cmd));
+  EXPECT_EQ(GL_NO_ERROR, GetGLError());
+  EXPECT_EQ(0u, GetAndClearBackbufferClearBitsForTest());
+}
+
 TEST_P(GLES2DecoderManualInitTest, DiscardFramebufferEXT) {
   InitState init;
   init.extensions = "GL_EXT_discard_framebuffer";