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 WillAccessBoundFramebufferForRead() {
    if (ShouldDeferReads())
      return error::kDeferCommandUntilLater;
    if (!offscreen_target_frame_buffer_.get() &&
        !framebuffer_state_.bound_read_framebuffer.get() &&
        !surface_->SetBackbufferAllocation(true))
      return error::kLostContext;
    return error::kNoError;
  }

  error::Error WillAccessBoundFramebufferForRead() {
    if (ShouldDeferReads())
      return error::kDeferCommandUntilLater;
    if (!offscreen_target_frame_buffer_.get() &&
        !framebuffer_state_.bound_read_framebuffer.get() &&
        !surface_->SetBackbufferAllocation(true))
      return error::kLostContext;
    return error::kNoError;
  }
