Make Image::image_observer_ WeakPersistent

Make Image::image_observer_ WeakPersistent

|image_observer_| was made UntracedMember during Oilpanization in [1],
and this CL turns it WeakPersistent, to clear |image_observer_|
when the corresponding ImageResourceContent is garbage collected while
Image is still alive because the Image is referenced from other than
ImageResourceContent.

This CL also add null checks of GetImageObserver() to SVG-related code,
because GetImageObserver() can be null since before.

[1] https://codereview.chromium.org/1610883002

BUG=726220
Review-Url: https://codereview.chromium.org/2905833003
Cr-Original-Commit-Position: refs/heads/master@{#475116}
Review-Url: https://codereview.chromium.org/2908333002 .
Cr-Commit-Position: refs/branch-heads/3112@{#24}
Cr-Branched-From: b6460e24cf59f429d69de255538d0fc7a425ccf9-refs/heads/master@{#474897}
Committed: https://chromium.googlesource.com/chromium/src/+/6c28cbc115d39a06d728f09c69bb7100f6aaec86

[hiroshige, 2017-05-30 08:19:11]

Description was changed from

==========
Make Image::image_observer_ WeakPersistent

|image_observer_| was made UntracedMember during Oilpanization in [1],
and this CL turns it WeakPersistent, to clear |image_observer_|
when the corresponding ImageResourceContent is garbage collected while
Image is still alive because the Image is referenced from other than
ImageResourceContent.

This CL also add null checks of GetImageObserver() to SVG-related code,
because GetImageObserver() can be null since before.

[1] https://codereview.chromium.org/1610883002

BUG=726220

Review-Url: https://codereview.chromium.org/2905833003
Cr-Commit-Position: refs/heads/master@{#475116}
(cherry picked from commit e056249f434f9ccca8024246ebb879d225b9f55a)
==========

to

==========
Make Image::image_observer_ WeakPersistent

|image_observer_| was made UntracedMember during Oilpanization in [1],
and this CL turns it WeakPersistent, to clear |image_observer_|
when the corresponding ImageResourceContent is garbage collected while
Image is still alive because the Image is referenced from other than
ImageResourceContent.

This CL also add null checks of GetImageObserver() to SVG-related code,
because GetImageObserver() can be null since before.

[1] https://codereview.chromium.org/1610883002

BUG=726220

Review-Url: https://codereview.chromium.org/2905833003
Cr-Original-Commit-Position: refs/heads/master@{#475116}
Review-Url: https://codereview.chromium.org/2908333002 .
Cr-Commit-Position: refs/branch-heads/3112@{#24}
Cr-Branched-From:
b6460e24cf59f429d69de255538d0fc7a425ccf9-refs/heads/master@{#474897}
Committed:
https://chromium.googlesource.com/chromium/src/+/6c28cbc115d39a06d728f09c69bb...
==========

[hiroshige, 2017-05-30 08:19:12]

Committed patchset #1 (id:1) manually as
6c28cbc115d39a06d728f09c69bb7100f6aaec86.