Ban variable shadowing in blink/platform

The compiler can warn if you are using several variables with the same
name, where one shadows another. This can be harmless or the sign of
code that is hard to read and even potentially has bugs now or in the
future.

This enables the shadowing warning for blink/platform, which will
implicitly ban variable shadowing there.

Bug: 925310
Change-Id: Id49fcd57f92ca188fa11ccccc7c16a524c84f4f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1730422
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#683254}