DevTools: Replace all untyped accesses to TimelineEvent.

+ remove a TimelineEvent hack from CodeGeneratorInspector.py

Review URL: https://codereview.chromium.org/116073007

git-svn-id: svn://svn.chromium.org/blink/trunk@164335 bbb929c8-8fbe-4397-9dbb-9b2b20218538