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

This contains fixes for the following sorts of issues:
* Possibly-uninitialized local variable
* Signedness mismatch
* "inline" invalid when friend declaration refers to a function template
specialization

BUG=81439
TEST=none

Review URL: https://codereview.chromium.org/390523002

git-svn-id: svn://svn.chromium.org/blink/trunk@177991 bbb929c8-8fbe-4397-9dbb-9b2b20218538