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
 void Framebuffer::MarkAttachmentAsCleared(
       RenderbufferManager* renderbuffer_manager,
       TextureManager* texture_manager,
      GLenum attachment,
      bool cleared) {
  AttachmentMap::iterator it = attachments_.find(attachment);
  if (it != attachments_.end()) {
    Attachment* a = it->second.get();
    if (a->cleared() != cleared) {
      a->SetCleared(renderbuffer_manager,
                    texture_manager,
                    cleared);
    }
  }
}

 void Framebuffer::MarkAttachmentAsCleared(
       RenderbufferManager* renderbuffer_manager,
       TextureManager* texture_manager,
      GLenum attachment,
      bool cleared) {
  AttachmentMap::iterator it = attachments_.find(attachment);
  if (it != attachments_.end()) {
    Attachment* a = it->second.get();
    if (a->cleared() != cleared) {
      a->SetCleared(renderbuffer_manager,
                    texture_manager,
                    cleared);
    }
  }
}
