Disable PDFExtensionTest.Metrics test with NetworkService

This new tests fails with the Network Service enabled just as other
PDFExtensionTests do. Lumping this in with issue 705114.

Bug: 705114
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I6da8f21354e7f809e8b86a3f57b3f6723e57e386
Tbr: jam@chromium.org
No-Try: true
No-Tree-Checks: true
No-Presubmit: true
Reviewed-on: https://chromium-review.googlesource.com/820810Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523236}

testing/buildbot/filters/mojo.fyi.network_browser_tests.filter

@@ -582,6 +582,7 @@
 -PDFExtensionTest.EnsureSameOriginRepliesAllowed
 -PDFExtensionTest.GestureDetector
 -PDFExtensionTest.LinkPermissions
+-PDFExtensionTest.Metrics
 -PDFExtensionTest.NavigationOnCorrectTab
 -PDFExtensionTest.Navigator
 -PDFExtensionTest.OpenFromFTP