Add command buffer support for GL_RGB CHROMIUM image emulation.

gpu/command_buffer/service/framebuffer_manager.cc

Index: gpu/command_buffer/service/framebuffer_manager.cc
diff --git a/gpu/command_buffer/service/framebuffer_manager.cc b/gpu/command_buffer/service/framebuffer_manager.cc
index ccd593aac225d52313867ba0472f7b9312fd0adc..3c4fad310464583503e3f51d026b214dacee0681 100644
--- a/gpu/command_buffer/service/framebuffer_manager.cc
+++ b/gpu/command_buffer/service/framebuffer_manager.cc
@@ -129,6 +129,8 @@ class RenderbufferAttachment
     return false;
   }
 
+  bool EmulatingRGB() const override { return false; }
+
  protected:
   ~RenderbufferAttachment() override {}
 
@@ -277,6 +279,10 @@ class TextureAttachment
     return texture == texture_ref_.get() && level == level_;
   }
 
+  bool EmulatingRGB() const override {
+    return texture_ref_->texture()->EmulatingRGB();
+  }
+
  protected:
   ~TextureAttachment() override {}
 
@@ -537,6 +543,10 @@ GLenum Framebuffer::GetReadBufferInternalFormat() const {
     return 0;
   }
   const Attachment* attachment = it->second.get();
+  if (attachment->EmulatingRGB()) {
+    DCHECK_EQ(static_cast<GLenum>(GL_RGBA), attachment->internal_format());
+    return GL_RGB;
+  }
   return attachment->internal_format();
 }