Access Isolate* and Heap* wherever already available.

Roughly:
GetIsolate(): -20
GetHeap(): -22
Handle<>(HeapObject): -315
handle(HeapObject): -21

Bug: v8:7786
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng;luci.v8.try:v8_linux_noi18n_rel_ng
Change-Id: I2da36ed1909d849812a1cb6bf94cb735eedca45b
Reviewed-on: https://chromium-review.googlesource.com/1111707
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53987}