Test coverage for excluding application/x-www-form-urlencoded from CORB.

In addition to covering application/x-www-form-urlencoded, this CL
takes the opportunity to also cover application/octet-stream which
was identified long time ago in a Firefox bug.

This CL is a follow-up to https://crbug.com/990853, but won't need to be
merged to release branches, so it doesn't refer to this bug below.

Bug: 802836
Change-Id: Idc95a1b070f4b4ecce391eb4f173196b89d989fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1739749
Commit-Queue: Charlie Reis <[email protected]>
Auto-Submit: Łukasz Anforowicz <[email protected]>
Reviewed-by: Charlie Reis <[email protected]>
Cr-Commit-Position: refs/heads/master@{#684988}

Author: anforowicz

fetch/corb/img-mime-types-coverage.tentative.sub.html

@@ -22,6 +22,13 @@
       "application/dash+xml", // video format
       "image/gif;HI=THERE",
 
+      // Non-image MIME-types that in practice get used for images on the web.
+      //
+      // https://bugzilla.mozilla.org/show_bug.cgi?id=1302539
+      "application/octet-stream",
+      // https://crbug.com/990853
+      "application/x-www-form-urlencoded",
+
       // MIME types that may seem to be JSON or XML, but really aren't - i.e.
       // these MIME types are not covered by:
       // - https://mimesniff.spec.whatwg.org/#json-mime-type