Add devtools trace event for event dispatching

Current implementation of InspectorTimelineAgent will emit EventDispatch event only if there is at least one handler on the event's path. After this patch EventDispather::dispatch trace events will be sent for unhandled events too. We should be able to filter out such events later on the front-end side and avoid additional checks at the instrumentation point.

BUG=361045

Review URL: https://codereview.chromium.org/239303003

git-svn-id: svn://svn.chromium.org/blink/trunk@171951 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Revert "Rebaseline layout tests after squashing is enabled."

This reverts r171946 (commit 249480dd6c5fe0b22d4e98c2f54f5935663e313c)
as the change it depends on is being reverted.

TBR=vollick@chromium.org
BUG=364860

Review URL: https://codereview.chromium.org/243083004

git-svn-id: svn://svn.chromium.org/blink/trunk@171953 bbb929c8-8fbe-4397-9dbb-9b2b20218538