Re-enable http/tests/misc/iframe-script-modify-attr.html

The test is passing with the changes made to the test harness.

R=avi@chromium.org

Bug: 801992, 801338, 1069111
Change-Id: If56cf2756e5ce272eb8619ca438f6fb04ee82be7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2218478Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772718}

third_party/blink/web_tests/TestExpectations

@@ -101,7 +101,6 @@ crbug.com/1050826 external/wpt/mixed-content/gen/top.http-rp/opt-in/object-tag.h
 # Tests temporarily disabled with Site Isolation - uninvestigated bugs:
 # TODO(lukasza, alexmos): Burn down this list.
 crbug.com/765779 http/tests/loading/bad-server-subframe.html [ Failure ]
-crbug.com/801992 http/tests/misc/iframe-script-modify-attr.html [ Pass Crash ]
 crbug.com/872952 http/tests/media/autoplay/document-user-activation-feature-policy-iframe-no-gesture.html [ Failure Pass Timeout ]
 crbug.com/895001 external/wpt/html/user-activation/message-event-activation-api-iframe-cross-origin.sub.tentative.html [ Timeout Pass ]
 crbug.com/895001 http/tests/media/autoplay/webaudio-autoplay-iframe-with-gesture.html [ Timeout Pass ]