[explicit isolates] Eliminate Handle<T>(T*) from api.cc

Explicitly pass Isolate* to the Handle constructor even if it means
calling GetIsolate. (Shortly, I want to remove that constructor
completely since its an invisible source of implicit GetIsolates).

Bug: v8:7786
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I3454e8c92e12bf5c5eb1d4e2018dff7c42949d26
Reviewed-on: https://chromium-review.googlesource.com/1089058
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53564}