Fix incorrectly disabled SSE2 WebAudio optimizations for Windows.

MSVC doesn't set __SSE__ or__SSE2__ defines:

http://msdn.microsoft.com/en-us/library/b0084kay.aspx

BUG=349320
TEST=compiles

Review URL: https://codereview.chromium.org/307963005

git-svn-id: svn://svn.chromium.org/blink/trunk@175173 bbb929c8-8fbe-4397-9dbb-9b2b20218538