Reland "Enable field trial testing config in content_unittests"

This is a reland of commit 4ac59628bf3b500cf5822b331d139d2ae7bce20f

Diff:
- Disable when using sanitizers, due to memory safety issues detected.
  Mostly invalid downcasts. I will fix them in a follow-up.
- Enable `AllowSwiftShaderFallback` to avoid a test to fail on mac.
- Revert line about https://crbug.com/447306958, because it has been
  fixed in between now and the revert.

--- Heads-up for the CQ sheriff: this patch is likely not to stick on
its very second attempt. If you see tests failing content_unittests,
please don't hesitate to revert this one in priority
---

Enable field trial testing config in content_unittests

When rolling out a new feature using field trials, Chrome engineers have
to edit the file testing/variations/fieldtrial_testing_config.json so
that their feature is somewhat properly tested before reaching
Dev/Beta/Stable users.

Currently, the field trial testing config is properly wired into
content_shell based tests like web tests, content_browsertest,
components_browsertests, etc...

However, coverage is missing in unittests.

This patch call the previously extracted function to do the proper
setup. We had to exclude some failing features.

Cq-Include-Trybots: luci.chromium.try:linux_chromium_asan_rel_ng
Cq-Include-Trybots: luci.chromium.try:linux_chromium_msan_rel_ng
Cq-Include-Trybots: luci.chromium.try:linux_chromium_tsan_rel_ng
Cq-Include-Trybots: luci.chromium.try:linux_chromium_ubsan_rel_ng
Cq-Include-Trybots: luci.chromium.try:mac13-arm64-rel
Cq-Include-Trybots: luci.chromium.try:mac14-arm64-rel
Cq-Include-Trybots: luci.chromium.try:mac15-arm64-rel
Bug: 40105939
Change-Id: Ieded8349c16031996839adb746c3bef9c2720da0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6989214
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Auto-Submit: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1523064}