[DevTools] [PowerProfiler] Fix for browser crash with active timeline recording for capturing power.
The PowerProfilerService is a singleton class. It stores the PowerProfilerObserver to notify
about the power events. With the active timeline recording, the DevToolsPowerHandler registers
itself as the observer. When the tab close event occurs, the DevToolsPowerHandler is killed.
But the PowerProfilerService still contains a dangling reference to the observer thereby
causing the crash.
With this CL, DevToolsPowerHandler will remove itself from the observer to avoid the dangling
reference with the PowerProfilerService.
BUG=383725
Review URL: https://codereview.chromium.org/
332623003
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@278798 0039d316-1c4b-4281-b951-d872f2087c98