Fix StrictMode violation in InstantSearchEngine

Get the current URL from the Tab rather than WebView to avoid calling
a WebView method on a background thread.

Bug: 5072398
Change-Id: I6e0fe0bac525fc772f659068dbc03511f20addf4
diff --git a/src/com/android/browser/InstantSearchEngine.java b/src/com/android/browser/InstantSearchEngine.java
index c913494..4dd83fa 100644
--- a/src/com/android/browser/InstantSearchEngine.java
+++ b/src/com/android/browser/InstantSearchEngine.java
@@ -199,7 +199,7 @@
             return false;
         }
 
-        final String currentUrl = current.getUrl();
+        final String currentUrl = mController.getCurrentTab().getUrl();
 
         if (currentUrl != null) {
             Uri uri = Uri.parse(currentUrl);