GitLab

This constant was only set to false on OS_FUCHSIA and has been
shipping as true for several years on all other platforms.

Cleans up the code to not need this constant in most places and
adds a OS_FUCHSIA ifdef around InstantiateFieldTrialAllocatorIfNeeded()
which is the only place that needed custom logic to disable it for
Fuchsia.

Bug: 867558
Change-Id: Idbf49ce571a50d67debeafe2cdacee086ebca537
Reviewed-on: https://chromium-review.googlesource.com/c/1343020
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609922}