Fixes for re-enabling more MSVC level 4 warnings: Source/ edition

This contains fixes for the following sorts of issues:
* Assignment inside conditional
* Signedness mismatch
* Possibly-uninitialized local variable

BUG=81439
TEST=none

Review URL: https://codereview.chromium.org/383303002

git-svn-id: svn://svn.chromium.org/blink/trunk@178100 bbb929c8-8fbe-4397-9dbb-9b2b20218538