Fix null pointer dereference in PLSA's scrollbarStyleSource.

Fix null pointer dereference in PLSA's scrollbarStyleSource.

The scroller inside the alt-text shadow tree created by HTMLImageFallbackHelper
was trying to read scrollbar styles from the <img> element, which had no
LayoutObject because it was styled with "display: contents".

BUG=697247
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2734993002
Cr-Commit-Position: refs/heads/master@{#455148}
(cherry picked from commit d23ef9b37d71d31b82613f8ddb8da614cd54c665)

Review-Url: https://codereview.chromium.org/2736873006 .
Cr-Commit-Position: refs/branch-heads/3029@{#71}
Cr-Branched-From: 939b32ee5ba05c396eef3fd992822fcca9a2e262-refs/heads/master@{#454471}
Committed: https://chromium.googlesource.com/chromium/src/+/bff179ffca3cc9e47629a280d8ffd40970bb2b38

[skobes, 2017-03-08 22:28:13]

Description was changed from

==========
Fix null pointer dereference in PLSA's scrollbarStyleSource.

The scroller inside the alt-text shadow tree created by HTMLImageFallbackHelper
was trying to read scrollbar styles from the <img> element, which had no
LayoutObject because it was styled with "display: contents".

BUG=697247
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2734993002
Cr-Commit-Position: refs/heads/master@{#455148}
(cherry picked from commit d23ef9b37d71d31b82613f8ddb8da614cd54c665)
==========

to

==========
Fix null pointer dereference in PLSA's scrollbarStyleSource.

The scroller inside the alt-text shadow tree created by HTMLImageFallbackHelper
was trying to read scrollbar styles from the <img> element, which had no
LayoutObject because it was styled with "display: contents".

BUG=697247
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2734993002
Cr-Commit-Position: refs/heads/master@{#455148}
(cherry picked from commit d23ef9b37d71d31b82613f8ddb8da614cd54c665)

Review-Url: https://codereview.chromium.org/2736873006 .
Cr-Commit-Position: refs/branch-heads/3029@{#71}
Cr-Branched-From:
939b32ee5ba05c396eef3fd992822fcca9a2e262-refs/heads/master@{#454471}
Committed:
https://chromium.googlesource.com/chromium/src/+/bff179ffca3cc9e47629a280d8ff...
==========

[skobes, 2017-03-08 22:28:14]

Committed patchset #1 (id:1) manually as
bff179ffca3cc9e47629a280d8ffd40970bb2b38 (tree was closed).