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WebViewPlugin didn't have the implementation of
blink::mojo::WidgetHost, it was able to access the null pointer
access when the mouse pointer is moved to on the plugin view, so
this CL implements blink::mojo::WidgetHost in WebViewPlugin and
adds WebWidget::GetTooltipText for the web test.

Fuzzer report: https://clusterfuzz.com/testcase?key=5128177771413504

Bug: 1081999, 1082648
Change-Id: Ia24d71b687850d796bff02bcca496dc04217c17d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2198517Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Miyoung Shin <myid.shin@igalia.com>
Cr-Commit-Position: refs/heads/master@{#769952}