Revert of Move Event to oilpan's heap (https://codereview.chromium.org/208173004/)

Reason for revert:
Seems likely to have caused a number of flaky failures in layout tests in oilpan builds:

Flakiness dashboard link: http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Blink&tests=http%2Ftests%2Fmedia%2Fmedia-source%2Fwebkitmediasource-closed-on-htmlmediaelement-destruction.html%2Csvg%2Fanimations%2Fsmil-leak-element-instances-noBaseValRef.svg%2Csvg%2Fanimations%2Fsmil-leak-element-instances.svg%2Csvg%2Fanimations%2Fsmil-leak-elements.svg

Example failure bot: http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Linux%20Oilpan/builds/2770

Example failure:
--- /mnt/data/b/build/slave/WebKit_Linux_Oilpan/build/layout-test-results/svg/animations/smil-leak-element-instances-noBaseValRef-expected.txt
+++ /mnt/data/b/build/slave/WebKit_Linux_Oilpan/build/layout-test-results/svg/animations/smil-leak-element-instances-noBaseValRef-actual.txt
@@ -1 +1 @@
-PASS
+FAIL: -3 extra live node(s)




Original issue's description:
> Move Event to oilpan's heap
> 
> Now there is no test failing when I change RefCounted<Event> to RefCountedGarbageCollected<Event>. Thus this CL does the change.
> 
> BUG=340522
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=169941

TBR=oilpan-reviews@chromium.org,ager@chromium.org,wibling@chromium.org,haraken@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=340522

Review URL: https://codereview.chromium.org/213403004

git-svn-id: svn://svn.chromium.org/blink/trunk@170092 bbb929c8-8fbe-4397-9dbb-9b2b20218538