<webview>: Fix and re-enable WebViewInteractiveTest.DragDropWithinWebView.
interactive_ui_tests seems to send addition clicks/(mouseup then down)
  sometimes. This causes the ongoing drag and drop to fail and the test
  to timeout.
This change detects the weird clicks and restarts the dnd operation.

This change also removes the use of TestServer() to serve guest pages and uses executeScript() instead.

BUG=161112
Test=Ran in trybot 600 times w/o failure
Sample run:
http://build.chromium.org/p/tryserver.chromium/builders/linux_chromeos/builds/163479

Review URL: https://codereview.chromium.org/26538010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229291 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229291