VR: Have testapp DCHECK against GL errors in UI code

Unbinding of frames appears to generate GL errors, on the first frame,
but otherwise, the UI is clean.  Checking for GL errors in the testapp
is a way to sanity check against new UI errors without affecting GPU
scheduling on dev/release Android builds.

BUG=768905
R=vollick

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_vr
Change-Id: I7ba27f20e59f4eb642f129795abb74d66a0894f5
Reviewed-on: https://chromium-review.googlesource.com/1070309Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: Christopher Grant <cjgrant@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562962}

chrome/browser/vr/testapp/gl_renderer.cc

@@ -20,6 +20,13 @@ void OnPresentedFrame(const gfx::PresentationFeedback& feedback) {
   // Do nothing for now.
 }
 
+bool ClearGlErrors() {
+  bool errors = false;
+  while (glGetError() != GL_NO_ERROR)
+    errors = true;
+  return errors;
+}
+
 }  // namespace
 
 GlRenderer::GlRenderer(const scoped_refptr<gl::GLSurface>& surface,
@@ -50,10 +57,18 @@ bool GlRenderer::Initialize() {
 void GlRenderer::RenderFrame() {
   context_->MakeCurrent(surface_.get());
 
+  // Checking and clearing GL errors can be expensive, but we can afford to do
+  // this in the testapp as a sanity check.  Clear errors before drawing UI,
+  // then assert no new errors after drawing.  See https://crbug.com/768905.
+  ClearGlErrors();
+
   glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
   glClear(GL_COLOR_BUFFER_BIT);
 
   vr_->DrawFrame();
+
+  DCHECK(!ClearGlErrors());
+
   PostRenderFrameTask(
       surface_->SwapBuffers(base::BindRepeating(&OnPresentedFrame)));
 }