Prevent crash on back button press if tab model not initialized

WebappActivity inflates its UI asynchronously thus the tab model may not
be initialized during early startup. Calling
ChromeActivity.getCurrentWebContents (happens on back button press),
would crash chrome if the Tab Model is not initialized yet. This cl
fixes the issue and makes ChromeActivity.getCurrentWebContents return
null instead.

Bug: 905583
Change-Id: I4add9cc2f41bb2544e279dd597b7b58a2f68b374
Reviewed-on: https://chromium-review.googlesource.com/c/1340917Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Commit-Queue: Mohamed Heikal <mheikal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608962}

chrome/android/java/src/org/chromium/chrome/browser/ChromeActivity.java

@@ -1750,6 +1750,9 @@ public abstract class ChromeActivity<C extends ChromeActivityComponent>
      *         WebContents.
      */
     public WebContents getCurrentWebContents() {
+        if (!mTabModelsInitialized) {
+            return null;
+        }
         return TabModelUtils.getCurrentWebContents(getCurrentTabModel());
     }