Remove focus on back key press if focus is on url bar.

Handlind back key event and clearing focus of url bar
and retaining focus to content view.

BUG=

Review URL: https://codereview.chromium.org/540853002

Cr-Commit-Position: refs/heads/master@{#294105}

content/shell/android/java/src/org/chromium/content_shell/Shell.java

@@ -166,6 +166,16 @@ public class Shell extends LinearLayout {
                 }
             }
         });
+        mUrlTextView.setOnKeyListener(new OnKeyListener() {
+            @Override
+            public boolean onKey(View v, int keyCode, KeyEvent event) {
+                if (keyCode == KeyEvent.KEYCODE_BACK) {
+                    mContentViewCore.getContainerView().requestFocus();
+                    return true;
+                }
+                return false;
+            }
+        });
     }
 
     /**