Fix invalid C-style cast in RenderFrameImplTest.
It casts (like reinterpret_cast) from one unrelated type
(content::RenderView*) to another unrelated type
(content::RenderWidget*). The result pointer is invalid,
but it's only used in EXPECT_NE, which would always be satisfied.
Using the correct static cast instead.
The bug was found by Control Flow Integrity check:
https://sites.google.com/a/chromium.org/dev/developers/testing/control-flow-integrity
BUG=chromium:457523,chromium:517544
Review URL: https://codereview.chromium.org/
1273183002
Cr-Commit-Position: refs/heads/master@{#342222}