Explain the reason why we're using isSimpleOrForbiddenRequest() in makeCrossOriginAccessRequest()

This change was made by https://codereview.chromium.org/379113002
Explain the reason in a comment for readability.

R=sigbjornf
BUG=none

Review URL: https://codereview.chromium.org/675993003

git-svn-id: svn://svn.chromium.org/blink/trunk@184368 bbb929c8-8fbe-4397-9dbb-9b2b20218538