[android_webview] Use a MotionEvent to trigger ShouldInterceptRequest tests.

This updates the test case to use a synthesized single tap gesture
to follow the link instead of JavaScript.

BUG=408426
TEST=AndroidWebViewTest

Review URL: https://codereview.chromium.org/552813002

Cr-Commit-Position: refs/heads/master@{#293926}
3 files changed