Mark content_shell_crash_test experimental on Win10 Tests x64 (dbg).

It's quite flaky.

TBR=martiniss@chromium.org
BUG=861730

Change-Id: Id1ec406b75c54451244976108fd44fc1937a1419
Reviewed-on: https://chromium-review.googlesource.com/1132600Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574045}

testing/buildbot/chromium.win.json

@@ -1928,6 +1928,7 @@
     ],
     "isolated_scripts": [
       {
+        "experiment_percentage": 100,
         "isolate_name": "content_shell_crash_test",
         "name": "content_shell_crash_test",
         "swarming": {

testing/buildbot/test_suite_exceptions.pyl

@@ -830,6 +830,12 @@
       # chromium.linux
       'Linux Tests (dbg)(1)(32)',  # https://crbug.com/859264
     ],
+    'modifications': {
+      # chromium.win
+      'Win10 Tests x64 (dbg)': {
+        'experiment_percentage': 100,  # https://crbug.com/861730
+      },
+    },
   },
   'content_shell_test_apk': {
     'remove_gtest_from': [