Consider "mixed content XHR" as mixed script instead of mixed display.

Second attempt.  The first attempt at https://codereview.chromium.org/14619021/
was reverted because it was over-aggressive: it could not distinguish between
raw XHR requests and other RAW (audio/video) request.

This CL adds a new flag to ResourceLoaderOptions to distinguish the two
cases, and check this in ResourceFetcher.cpp.  DocumentThreadableLoader's
sync case is then made to use the new checking code in ResourceFetcher in
place of its own check.

BUG=239540

Review URL: https://chromiumcodereview.appspot.com/23437013

git-svn-id: svn://svn.chromium.org/blink/trunk@157139 bbb929c8-8fbe-4397-9dbb-9b2b20218538