Correct comment in bindings template

Comment-only change.

An existing comment explains some code as saying "necessary
for MSVC", but it's actually required by the C++ standard.

This CL corrects the comment and explains.

R=haraken

Review URL: https://codereview.chromium.org/331783003

git-svn-id: svn://svn.chromium.org/blink/trunk@176075 bbb929c8-8fbe-4397-9dbb-9b2b20218538