WebView: disable Safe Browsing test cases

These tests all exhibit flakiness but we don't have time right now to
debug them. Mark them as @Ignore in hopes that Android infra stops
nagging us about the issue.

Bug: 40196091
Test: atest CtsWebkitTestCases:WebViewClientTest
Flag: TEST_ONLY
Change-Id: If48ceca70ec18b51b2803bd926d44080e353bf18