Add a use-counter for viewport-fixed position:fixed under a filter

Add a use-counter for viewport-fixed position:fixed under a filter

This will inform whether we can make the change suggested in
https://github.com/w3c/fxtf-drafts/issues/11

BUG=711736
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2817583006
Cr-Commit-Position: refs/heads/master@{#464853}
(cherry picked from commit e208bf05c117cf2fa0f7aab01a779d47fd92b8ef)

Review-Url: https://codereview.chromium.org/2821943002 .
Cr-Commit-Position: refs/branch-heads/3071@{#14}
Cr-Branched-From: a106f0abbf69dad349d4aaf4bcc4f5d376dd2377-refs/heads/master@{#464641}
Committed: https://chromium.googlesource.com/chromium/src/+/13bf399cb6cfe09f5492834f07b1bd6bf8d0ae44

[chrishtr, 2017-04-17 16:02:37]

Description was changed from

==========
Add a use-counter for viewport-fixed position:fixed under a filter

This will inform whether we can make the change suggested in
https://github.com/w3c/fxtf-drafts/issues/11

BUG=711736
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2817583006
Cr-Commit-Position: refs/heads/master@{#464853}
(cherry picked from commit e208bf05c117cf2fa0f7aab01a779d47fd92b8ef)
==========

to

==========
Add a use-counter for viewport-fixed position:fixed under a filter

This will inform whether we can make the change suggested in
https://github.com/w3c/fxtf-drafts/issues/11

BUG=711736
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2817583006
Cr-Commit-Position: refs/heads/master@{#464853}
(cherry picked from commit e208bf05c117cf2fa0f7aab01a779d47fd92b8ef)

Review-Url: https://codereview.chromium.org/2821943002 .
Cr-Commit-Position: refs/branch-heads/3071@{#14}
Cr-Branched-From:
a106f0abbf69dad349d4aaf4bcc4f5d376dd2377-refs/heads/master@{#464641}
Committed:
https://chromium.googlesource.com/chromium/src/+/13bf399cb6cfe09f5492834f07b1...
==========

[chrishtr, 2017-04-17 16:02:38]

Committed patchset #1 (id:1) manually as
13bf399cb6cfe09f5492834f07b1bd6bf8d0ae44.