NavigationLoaderOnUI: Disable failing sign-in browser tests.

GcmApiTest.* tests are failing at
HeaderModificationDelegateOnUIThreadImpl::ProcessRequest when
NavigationLoaderOnUI is enabled.

Bug: 978617
Change-Id: Ie483c0b392a4c706848dbbcb14726b31c82f8e72
TBR: kinuko
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1714392Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679898}

testing/buildbot/filters/navigation_loader_on_ui_browser_tests.filter

 # These tests currently fail when run with --enable-features=NavigationLoaderOnUI
+
+# https://crbug.com/978617
+-GcmApiTest.*