Don't trigger BackForwardCache field trial group when checking LoadingTasksUnfreezable

Currently we check the LoadingTasksUnfreezable by directly calling
base::FeatureList::IsEnabled(features::kLoadingTasksUnfreezable). This
has a side effect of activating the current user to be counted in the
BackForwardCache field trial. We do not want this to happen when
back-forward cache is disabled for that user e.g. due to low system RAM
as it will make this ineligible users to be counted for our hit rate,
etc.

This CL replaces all direct calls to base::FeatureList::IsEnabled() for
the kLoadingTasksUnfreezable feature to be guarded by a back-forward
cache feature check, to ensure we won't accidentally activate the
field trial for an ineligible user

Note that calling RuntimeEnabledFeatures::BackForwardCacheEnabled()
won't result in any accidental activation.

Bug: 1137682
Change-Id: I1f7c39d8671ef2b8178d6ccbf1e4c2241c5ca2ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2670604


Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#850439}