Move ui_test_utils::CrashTab to browser_test_utils so that it can be reused by content_browsertests.

content/public/test/browser_test_utils.h

Index: content/public/test/browser_test_utils.h
===================================================================
--- content/public/test/browser_test_utils.h	(revision 148487)
+++ content/public/test/browser_test_utils.h	(working copy)
@@ -53,6 +53,9 @@
 // tab is currently web_contents.  Otherwise returns immediately.
 void WaitForLoadStop(WebContents* web_contents);
 
+// Causes the specified web_contents to crash. Blocks until it is crashed.
+void CrashTab(WebContents* web_contents);
+
 // Simulates clicking at the center of the given tab asynchronously.
 void SimulateMouseClick(WebContents* web_contents);