Validate TestRenderViewHost downcast in TestWebContents.

crrev.com/c/1515904 managed to land with an invalid downcast in
TestWebContents::GetRenderViewHost which was only caught by the CFI bot.
This patch adds some manual RTTI and checks the cast, to avoid a future
test using TestWebContents without the right dependencies.

Change-Id: I40cd2a9ae22cc611f94a5b11050d1eb06f428c6d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529912
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#643610}
5 files changed