The lifetime of the collator is handled by the JavaScript heap. At the
moment this is implemented with a weak GlobalHandle. With this CL I
change the implementation to use a Managed object instead. In addition I
did some code cleanup.

The main reason for using a Managed is an lsan problem. The final GC in
d8 is triggered before all pending WebAssembly compilations get
canceled. Via the native context, WebAssembly compilation can keep the
Collator wrapper alive, and therefore the collator is never deallocated.
Managed, however, get processed at isolate teardown, independent of the
reachability of the Managed.

TEST=mjsunit/regress/regress-813440

Bug: chromium:813440
Cq-Include-Trybots: luci.v8.try:v8_linux_noi18n_rel_ng
Change-Id: Ie727eb1aff2144586eb36426cc44a32357c0f822
Reviewed-on: https://chromium-review.googlesource.com/956069
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51886}