CTS : modify "loadURL" method that wait until getProgress returns 100.

CTS : modify "loadURL" method that wait until getProgress returns 100.

- It appears that the updates returned by getProgress are not accurate
unless there is a WebChromeClient set on the WebView as seen in the code
of CallbackProxy#onProgressChanged.

refer to :https://review.source.android.com//#change,22046

Change-Id: I21991f70b40d917dcf3d785269f600965d603550