Merge WaitForLoadUrl into WebViewOnUiThread.

 Bug 5788999
 Make it more difficult to make mistakes with
 WaitForLoadUrl.waitForLoadComplete by merging with
 WebViewOnUiThread. WebViewOnUiThread has helpers
 that do loading and waiting. The waitForLoadCompletion
 comes with a warning not to use it alone.

 Removing WaitForLoadUrl moves WebViewTest to use
 WebViewOnUiThread and many tests were simplified
 by putting UI thread calls into WebViewOnUiThread.

Change-Id: I064d9e4b704f3f9788a077ec46ed6a21fff0c403
8 files changed