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}
[turbofan] Minor cleanup to reduce code duplication.

TEST=unittests
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/819103002

Cr-Commit-Position: refs/heads/master@{#25927}
Remove custom weak handling of monomorphic IC.

This is not needed anymore since all ICs use weak cells to embed maps.

BUG=v8:3629
LOG=N

Review URL: https://codereview.chromium.org/817223003

Cr-Commit-Position: refs/heads/master@{#25928}
Mark mjsunit/setters-on-elements as flaky

TBR=machenbach@chromium.org
LOG=n
BUG=v8:3784
NOTREECHECKS=true

Review URL: https://codereview.chromium.org/817293002

Cr-Commit-Position: refs/heads/master@{#25929}