Marking extensions_browsertest suite as experimental on Win10 x64 DBG

There are several bugs tracking falky tests on this buildbot and this
suite is yet another which has mutliple tests timing out w/ no
discernable cause.  Marking experimental so sheriffs do not need to
retriage/snooze this issue each time the flake occurs.

Bug: 876615, 840369

Change-Id: I9c8bd562a32ef352322f27a93a58c45dd091d91e
Reviewed-on: https://chromium-review.googlesource.com/c/1289574Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600949}

testing/buildbot/chromium.win.json

@@ -2518,6 +2518,7 @@
         "test": "events_unittests"
       },
       {
+        "experiment_percentage": 100,
         "swarming": {
           "can_use_on_swarming_builders": true,
           "dimension_sets": [

testing/buildbot/test_suite_exceptions.pyl

@@ -494,6 +494,13 @@
       'linux-win_cross-rel', # https://crbug.com/762167
     ],
   },
+  'extensions_browsertests': {
+    'modifications': {
+      'Win10 Tests x64 (dbg)': {
+        'experiment_percentage': 100, # https://crbug.com/876615
+      },
+    },
+  },
   'gin_unittests': {
     'remove_from': [
       # chromium.clang