android: fully enable content_browsertests on android-pie-arm64-rel

It runs 100% experimentally since crrev.com/c/2541222.
The build duration (http://shortn/_4qFyYutBjf) and
test devices (http://shortn/_gE18OBAzt8) are still in good shape.

Bug: 1111436
Change-Id: Iee624ad216243cd6a6b07afc9e64456acb5cdd60
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2544749Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: Haiyang Pan <hypan@google.com>
Commit-Queue: Haiyang Pan <hypan@google.com>
Cr-Commit-Position: refs/heads/master@{#828973}

testing/buildbot/chromium.android.json

@@ -39149,7 +39149,6 @@
           "--gs-results-bucket=chromium-result-details",
           "--recover-devices"
         ],
-        "experiment_percentage": 100,
         "merge": {
           "args": [
             "--bucket",

testing/buildbot/test_suite_exceptions.pyl

@@ -1057,9 +1057,6 @@
           '--gtest_filter=-BackgroundTracingManagerBrowserTest.PerfettoSystemBackgroundScenario*',
         ],
       },
-      'android-pie-arm64-rel': {
-        'experiment_percentage': 100,
-      },
       'android-pie-x86-rel': {
         'args': [
           '--test-launcher-filter-file=../../testing/buildbot/filters/android.emulator_p.content_browsertests.filter',