[WebView] Fix several WebView CTS tests (same root failure)

Fixes:
WebViewClientTest#testOnScaleChanged
WebViewTest#testSetWebViewClient

Load some content into the view so we can zoom it and wait for the
condition we are testing rather than assuming an idle sync is
enough.

Bug: 8271539
Change-Id: Ibb3a723d688168bc4837ec2240e6d20a4fc9e9c6