Cleanup BitmapImage

- remove unused include wtf/CurrentTime.h
- outline currentFrameHasSingleSecurityOrigin
- make BitmapImage::getHotSpot return the m_source result directly
- remove isAnimated() helper: its name is inaccurate since it only
  wants to check if the image has multiple frames, which is better
  done with a frameCount() > 1 test
- BitmapImage.h: make class member variables private, clarify the
  comment about lazy animation start / stop
- minor code format / placement tweaks (cf., trac.webkit.org)

No change in behavior, covered by existing tests.

BUG=

Review URL: https://codereview.chromium.org/586323003

git-svn-id: svn://svn.chromium.org/blink/trunk@182383 bbb929c8-8fbe-4397-9dbb-9b2b20218538