gpu: Correct GMB format for GL_RGB BackTexture.

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 8a035114f52e142bd7a88abe407e790945175b03..11fa18c43591223152e29ee31051df35a29bfa80 100644
--- a/gpu/command_buffer/service/gles2_cmd_decoder.cc
+++ b/gpu/command_buffer/service/gles2_cmd_decoder.cc
@@ -2779,10 +2779,13 @@ GLenum BackTexture::Target() {
 bool BackTexture::AllocateNativeGpuMemoryBuffer(const gfx::Size& size,
                                                 GLenum format,
                                                 bool zero) {
-  gfx::BufferFormat buffer_format = gfx::BufferFormat::RGBA_8888;
+  DCHECK(format == GL_RGB || format == GL_RGBA);
   scoped_refptr<gl::GLImage> image =
       decoder_->GetContextGroup()->image_factory()->CreateAnonymousImage(
-          size, buffer_format, format);
+          size,
+          format == GL_RGB ? gfx::BufferFormat::RGBX_8888
+                           : gfx::BufferFormat::RGBA_8888,
+          format);
   if (!image || !image->BindTexImage(Target()))
     return false;