Enable the embedder to specify what kind of context was disposed

Enable the embedder to specify what kind of context was disposed

This API is used by Blink to inform V8 about HTML frames being disposed.
Using the optional parameter, Blink can tell V8 whether the disposed
frame was a main frame. In that case, we might want to reset GC
parameters

BUG=none
R=hpayer@chromium.org
LOG=y

Committed: https://crrev.com/18b1e6d353501a30a750ac12da68bdc0f93b923e
Cr-Commit-Position: refs/heads/master@{#25926}

[Hannes Payer (out of office), 2014-12-22 13:58:25]

lgtm, thx

[Hannes Payer (out of office), 2014-12-22 13:59:41]

The CQ bit was checked by hpayer@chromium.org

[commit-bot: I haz the power, 2014-12-22 14:00:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/823583003/1

[commit-bot: I haz the power, 2014-12-22 14:27:28]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-12-22 14:27:45]

Patchset 1 (id:??) landed as
https://crrev.com/18b1e6d353501a30a750ac12da68bdc0f93b923e
Cr-Commit-Position: refs/heads/master@{#25926}