Add way to get native VR UI information from Java

chrome/browser/android/vr_shell/ui_scene.h

Index: chrome/browser/android/vr_shell/ui_scene.h
diff --git a/chrome/browser/android/vr_shell/ui_scene.h b/chrome/browser/android/vr_shell/ui_scene.h
index 4bcb6d88d29a490db659f71ee6a5720adfad48f4..1ddee18c58904f9ecd25d6170f477ed7818977be 100644
--- a/chrome/browser/android/vr_shell/ui_scene.h
+++ b/chrome/browser/android/vr_shell/ui_scene.h
@@ -6,6 +6,7 @@
 #define CHROME_BROWSER_ANDROID_VR_SHELL_UI_SCENE_H_
 
 #include <memory>
+#include <string>
 #include <vector>
 
 #include "base/macros.h"
@@ -67,6 +68,7 @@ class UiScene {
   const std::vector<std::unique_ptr<ContentRectangle>>& GetUiElements() const;
 
   ContentRectangle* GetUiElementById(int element_id);
+  ContentRectangle* GetUiElementByName(const std::string& element_name);
 
   std::vector<const ContentRectangle*> GetWorldElements() const;
   std::vector<const ContentRectangle*> GetHeadLockedElements() const;
@@ -76,6 +78,10 @@ class UiScene {
   float GetBackgroundDistance() const;
   bool GetWebVrRenderingEnabled() const;
 
+  // For testing whether the native state matches expectations

[cjgrant, 2017/03/30 14:48:35]

- Needs a period (all comments need to be full sentences as per style guide).
- Sorry to be picky, but could you further elaborate?  Something like "Builds a
dictionary of UI element properties for testing purposes. The dictionary can be
serialized and passed to Java in end-to-end tests."  Or something.

[bsheedy, 2017/03/30 21:05:14]

Done.

+  std::unique_ptr<base::DictionaryValue> CreateUiElementInfo(
+      std::string element_name);

[cjgrant, 2017/03/30 14:48:36]

const std::string& here too.  Could you scrub the rest of the CL and make sure
none are missed?  In general, any time you're passing an object for read-only
purposes, you want to do it as a const reference.  The style guide points out
that if you'll modify the object it should be passed through a pointer instead,
but that doesn't matter here.  

[bsheedy, 2017/03/30 21:05:14]

Done, found one additional place where const ref could be used (callback to the
main thread).

+
  private:
   void ApplyRecursiveTransforms(ContentRectangle* element);
   void ApplyDictToElement(const base::DictionaryValue& dict,