Framebuffer clear() needs to consider the situation some draw buffers are disabled.

This is when we expose DrawBuffers extension.

BUG=376951
TEST=the attached test case, webgl conformance
R=kbr@chromium.org,bajones@chromium.org

Review URL: https://codereview.chromium.org/315283002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@275338 0039d316-1c4b-4281-b951-d872f2087c98
error::Error GLES2DecoderImpl::HandleScheduleOverlayPlaneCHROMIUM(
    uint32 immediate_data_size,
    const cmds::ScheduleOverlayPlaneCHROMIUM& c) {
  NOTIMPLEMENTED() << "Overlay supported isn't finished.";
  LOCAL_SET_GL_ERROR(GL_INVALID_OPERATION,
                     "glScheduleOverlayPlaneCHROMIUM",
                     "function not implemented");
  return error::kNoError;
}

error::Error GLES2DecoderImpl::HandleScheduleOverlayPlaneCHROMIUM(
    uint32 immediate_data_size,
    const cmds::ScheduleOverlayPlaneCHROMIUM& c) {
  NOTIMPLEMENTED() << "Overlay supported isn't finished.";
  LOCAL_SET_GL_ERROR(GL_INVALID_OPERATION,
                     "glScheduleOverlayPlaneCHROMIUM",
                     "function not implemented");
  return error::kNoError;
}
