Move ImageObserver to platform/

BUG=297477

Review URL: https://codereview.chromium.org/60253002

git-svn-id: svn://svn.chromium.org/blink/trunk@161404 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Auto-rebaseline for r161389

http://src.chromium.org/viewvc/blink?view=revision&revision=161389

BUG=313327
TBR=leviw@chromium.org

Review URL: https://codereview.chromium.org/61423002

git-svn-id: svn://svn.chromium.org/blink/trunk@161405 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Auto-rebaseline for r161394

http://src.chromium.org/viewvc/blink?view=revision&revision=161394

BUG=315271
TBR=esprehn@chromium.org

Review URL: https://codereview.chromium.org/61263004

git-svn-id: svn://svn.chromium.org/blink/trunk@161406 bbb929c8-8fbe-4397-9dbb-9b2b20218538