Add a setDebugDirtyRegion() feature to the client.

Calling remoting.clientSession.setDebugDirtyRegion(true) enables
rendering of each frame's dirty region with an purple, translucent
overlay.

Currently the dirty region is re-rendered immediately for each frame,
with no linger nor fade-out behaviour.

BUG=427659

Review URL: https://codereview.chromium.org/932013002

Cr-Commit-Position: refs/heads/master@{#317496}
void PepperVideoRenderer3D::CheckGLError() {
  GLenum error = gles2_if_->GetError(graphics_.pp_resource());
  CHECK_EQ(error, static_cast<GLenum>(GL_NO_ERROR)) << "GL error: " << error;
}

void PepperVideoRenderer3D::CheckGLError() {
  GLenum error = gles2_if_->GetError(graphics_.pp_resource());
  CHECK_EQ(error, static_cast<GLenum>(GL_NO_ERROR)) << "GL error: " << error;
}
