Add way to get native VR UI information from Java

chrome/browser/resources/vr_shell/vr_shell_ui.js

Index: chrome/browser/resources/vr_shell/vr_shell_ui.js
diff --git a/chrome/browser/resources/vr_shell/vr_shell_ui.js b/chrome/browser/resources/vr_shell/vr_shell_ui.js
index 7203c2c9dbe58c76c98e1f4aa737e4ffca505a28..e77fc942ca2b8af11ae5c9382ad082cf36446c2e 100644
--- a/chrome/browser/resources/vr_shell/vr_shell_ui.js
+++ b/chrome/browser/resources/vr_shell/vr_shell_ui.js
@@ -16,6 +16,8 @@ var vrShellUi = (function() {
   // This value should match the one in VrShellImpl.java
   /** @const */ var UI_DPR = 1.2;
 
+  var domIdToUiElementIdMap = {};
+
   function getStyleFloat(style, property, defaultValue) {
     let value = parseFloat(style.getPropertyValue(property));
     return isNaN(value) ? defaultValue : value;
@@ -36,6 +38,10 @@ var vrShellUi = (function() {
         '%, ' + background + ' 100%)';
   }
 
+  function getUiElementId(domId) {
+    return domId in domIdToUiElementIdMap ? domIdToUiElementIdMap[domId] : -1;
+  }
+
   class ContentQuad {
     constructor() {
       /** @const */ this.SCREEN_HEIGHT = 1.375;
@@ -206,6 +212,7 @@ var vrShellUi = (function() {
 
       this.id = ui.addElement(element);
       this.domElement = domElement;
+      domIdToUiElementIdMap[domId] = this.id;

[tiborg, 2017/03/27 15:14:33]

cjgrant@: is this a good workaround until the names exist?

[cjgrant, 2017/03/27 15:41:32]

Brian said he'd wait on using element names, as that patch will land soon. 
Personally I'd prefer not to have this particular map, but either way, we will
need a "name to element" map.

[bsheedy, 2017/03/27 17:49:09]

Do we really need a map since we're removing all the JS from this CL once your
patch lands? On the native side, GetUiElementById just uses a linear search, so
I imagine GetUiElementByName will do the same.

[cjgrant, 2017/03/27 19:49:43]

That works if test will only ever check native state.  I can't imagine that
being the case though.  I picture tests checking a mix of native and Javascript
state, depending on what exactly you're testing.  But as always, I guess we add
what we need, when we need it.

[tiborg, 2017/03/27 19:59:59]

+1 to add the map once we need it.

     }
   };
 
@@ -1339,6 +1346,7 @@ var vrShellUi = (function() {
   return {
     initialize: initialize,
     command: command,
+    getUiElementId: getUiElementId,
   };
 })();