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

Review URL: https://codereview.chromium.org/823583003

Cr-Commit-Position: refs/heads/master@{#25926}