Previously, the serializer would share mutable heap numbers between
contexts. The workaround was to disable double field tracking entirely
during bootstrapping of isolates preparing to be serialized.

This does not cover custom scripts run between bootstrapping and
serialization, and can cause race conditions when writing to the flag.

This no longer seems necessary since we can correctly tell mutable and
immutable heap numbers apart by instance type now.

Bug: v8:6585
Change-Id: I7a59ffaad9d96f1c2b08813e19505f4fda95e555
Reviewed-on: https://chromium-review.googlesource.com/566861Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46591}