Currently there are a number of comment in src/v8.h which look like
this: TODO(dcarney): mark V8_WARN_UNUSED_RESULT.

This commit attempts to remove these comments and add the
V8_WARN_UNUSED_RESULT macro to the methods in question.

BUG=

Review-Url: https://codereview.chromium.org/2135973002
Cr-Commit-Position: refs/heads/master@{#44010}