Rename LifecycleObserver<T>::dispose() as clearContext()

Rename LifecycleObserver<T>::dispose() as clearContext()

Have the name of this helper match what it does; dispose() is too generic.
Observer instances only need to clear out their context and rarely, so
tighten up the visibility accordingly.

R=haraken
BUG=none

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=196502

[sof, 2015-06-04 12:12:40]

sigbjornf@opera.com changed reviewers:
+ haraken@chromium.org

[sof, 2015-06-04 12:12:41]

please take a look.

Resolving calls to dispose() from classes that inherit LifecycleObserver<> is
harder than it needs to be when reading code.

[haraken, 2015-06-04 12:44:00]

LGTM

[sof, 2015-06-04 12:50:58]

The CQ bit was checked by sigbjornf@opera.com

[commit-bot: I haz the power, 2015-06-04 12:51:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1160253006/20001

[commit-bot: I haz the power, 2015-06-04 13:56:42]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=196502