Merge 112549 - Web Inspector: subtree disapears from <iframe> after loading

Merge 112549 - Web Inspector: subtree disapears from <iframe> after loading
https://bugs.webkit.org/show_bug.cgi?id=76552

Reviewed by Yury Semikhatsky.

Source/WebCore:

The problem was that content document subtree was not unbound upon iframe re-push.
Upon owner element refresh content document was not sent to the frontend
since backend assumed that front-end has already had the up-to-date version.

Test: inspector/elements/iframe-load-event.html

* inspector/InspectorDOMAgent.cpp:
(WebCore::InspectorDOMAgent::loadEventFired):

LayoutTests:

* inspector/elements/iframe-load-event-expected.txt: Added.
* inspector/elements/iframe-load-event.html: Added.
* inspector/elements/resources/iframe-load-event-iframe-1.html: Added.
* inspector/elements/resources/iframe-load-event-iframe-2.html: Added.
* inspector/elements/resources/iframe-load-event-iframe.js: Added.
(loadSecondIFrame):
(test.step1.nodeInserted):
(test.step1):
(test.step2):
(test):

TBR=pfeldman@chromium.org
BUG=121116
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=112847