Add null checks of GetImageObserver() in BitmapImage

Add null checks of GetImageObserver() in BitmapImage

Because GetImageObserver() can be null but some callsites have used it
without null checks.
This CL adds null checks to BitmapImage, while [1] adds to SVG-related
classes.

[1] https://codereview.chromium.org/2905833003/

BUG=726220
Review-Url: https://codereview.chromium.org/2906583003
Cr-Original-Commit-Position: refs/heads/master@{#475147}
Review-Url: https://codereview.chromium.org/2914583003 .
Cr-Commit-Position: refs/branch-heads/3112@{#25}
Cr-Branched-From: b6460e24cf59f429d69de255538d0fc7a425ccf9-refs/heads/master@{#474897}
Committed: https://chromium.googlesource.com/chromium/src/+/bb2694f0cbf46f6ac6b467bf04a9373e4981c2ba

[hiroshige, 2017-05-30 09:34:52]

Description was changed from

==========
Add null checks of GetImageObserver() in BitmapImage

Because GetImageObserver() can be null but some callsites have used it
without null checks.
This CL adds null checks to BitmapImage, while [1] adds to SVG-related
classes.

[1] https://codereview.chromium.org/2905833003/

BUG=726220

Review-Url: https://codereview.chromium.org/2906583003
Cr-Commit-Position: refs/heads/master@{#475147}
(cherry picked from commit 1c33b530e841ad76be6e6c472711fda4e09c9d4d)
==========

to

==========
Add null checks of GetImageObserver() in BitmapImage

Because GetImageObserver() can be null but some callsites have used it
without null checks.
This CL adds null checks to BitmapImage, while [1] adds to SVG-related
classes.

[1] https://codereview.chromium.org/2905833003/

BUG=726220

Review-Url: https://codereview.chromium.org/2906583003
Cr-Original-Commit-Position: refs/heads/master@{#475147}
Review-Url: https://codereview.chromium.org/2914583003 .
Cr-Commit-Position: refs/branch-heads/3112@{#25}
Cr-Branched-From:
b6460e24cf59f429d69de255538d0fc7a425ccf9-refs/heads/master@{#474897}
Committed:
https://chromium.googlesource.com/chromium/src/+/bb2694f0cbf46f6ac6b467bf04a9...
==========

[hiroshige, 2017-05-30 09:34:53]

Committed patchset #1 (id:1) manually as
bb2694f0cbf46f6ac6b467bf04a9373e4981c2ba.