Revert obsolete use counter.

It's obsolete because the CSSWG resolved to fix ths issue by special-casing
the root element. See w3c/fxtf-drafts#11

TBR=trchen

Bug: 711736
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: Ie17644b5d8d2fd8f34e97ee0803b26854ef0150d
Reviewed-on: https://chromium-review.googlesource.com/831399
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#525581}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: d35b3dbaf9b11af8fe30ae37098b5381635b8d37

metrics/histograms/enums.xml

@@ -16948,7 +16948,7 @@ Called by update_net_error_codes.py.-->
   <int value="1958" label="MediaStreamConstraintsGoogDAEchoCancellation"/>
   <int value="1959" label="MediaStreamConstraintsGoogNoiseReduction"/>
   <int value="1960" label="MediaStreamConstraintsGoogPowerLineFrequency"/>
-  <int value="1961" label="ViewportFixedPositionUnderFilter"/>
+  <int value="1961" label="ViewportFixedPositionUnderFilter (obsolete)"/>
   <int value="1962"
       label="RequestMIDIAccessWithSysExOption_ObscuredByFootprinting"/>
   <int value="1963"