[MTE] Fix blink_heap_unittests failure on android-mte-arm64.

blink_heap_unittests crashes on android-mte-arm64 bot, because:
- ShouldEnableMemoryTaggingInRenderer() has DCHECK(
  FeatureList::GetInstance).
- ScopedUnittestsEnvironmentSetup's constructor invokes
  ShouldEnableMemoryTaggingInRenderer() inside
  WTF::Partitions::Initialize().
- ScopedUnittestsEnvironmentSetup is created before
  FeatureListScopedToEachTest::OnTestStart().

To avoid the DCHECK faiure, need ScopedFeatureList before
WTF::Partitions::Initialize().

Change-Id: Iaffa9d2813b3e72affe77f466b21578dc8c66a19
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5730255
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Commit-Queue: Takashi Sakamoto <tasak@google.com>
Cr-Commit-Position: refs/heads/main@{#1332764}