WebView: make evaluateJavascript helper more async

No change to test logic.

This edits an async helper method to call the async
WebkitUtils.onMainThread() method. onMainThreadSync() does extra
synchronization we don't need.

Bug: 127521788
Test: atest CtsWebkitTestCases:WebViewTest#testEvaluateJavascript
Change-Id: Iaf99eaad38e2e2981c826021d28f4930f6802a0f
1 file changed