To make warning flags between Windows and non-Windows clang consistent,
I'd like to build chromium_code targets with -Wextra enabled.  This fixes
-Wextra warnings in about 14k of 31k build steps.

Most of the warnings were about signed/unsigned mismatches; some were about
"const"s that had no effect.

(This CL does not contain the change to actually turn on -Wextra, only
cleanups.)

BUG=567877
TBR=content/common/sandbox

Review URL: https://codereview.chromium.org/1507413003

Cr-Commit-Position: refs/heads/master@{#364232}