[CRD iOS] Fallback to GLES2 when GLES3 is not supported

The percentage of seeing a GLES2-only device is low but it's still
safer to handle that, given that doing so requires only a few more lines.

Bug: 787224
Change-Id: I63650f3ce4c2afe93492b81e30d7075d7007ebdc
Reviewed-on: https://chromium-review.googlesource.com/780801Reviewed-by: Jamie Walch <jamiewalch@chromium.org>
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518363}

remoting/ios/display/gl_display_handler.mm

@@ -119,10 +119,15 @@ void Core::Initialize() {
 
   eagl_context_ = [EAGLContext currentContext];
   if (!eagl_context_) {
-    // TODO(nicholss): For prod code, make sure to check for ES3 support and
-    // fall back to ES2 if needed.
     eagl_context_ =
         [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES3];
+    if (!eagl_context_) {
+      LOG(WARNING) << "Failed to create GLES3 context. Atempting to create "
+                   << "GLES2 context.";
+      eagl_context_ =
+          [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2];
+    }
+    DCHECK(eagl_context_);
     [EAGLContext setCurrentContext:eagl_context_];
   }
 
@@ -130,10 +135,6 @@ void Core::Initialize() {
 
   renderer_proxy_->Initialize(renderer_->GetWeakPtr());
 
-  //  renderer_.RequestCanvasSize();
-
-  // demo_screen_ = new GlDemoScreen();
-  // renderer_->AddDrawable(demo_screen_->GetWeakPtr());
   renderer_->SetDelegate(weak_ptr_);
 }