Set request context when restoring cached CSSFontFaceSrcValue resource

This patch changes the resource context type of the FetchRequest issued
in CSSFontFaceSrcValue::restoreCachedResourceIfNeeded from
WebURLRequest::RequestContextUnspecified to
WebURLRequest::RequestContextFont.

BUG=624474

Review-Url: https://codereview.chromium.org/2162913002
Cr-Commit-Position: refs/heads/master@{#406805}
4 files changed