Remove dead code in NativeImageSkia.

Also rename its SkBitmap member m_bitmap, which is both more consistent
with its accessor and with its type (m_image seems to suggest that it is
a blink::Image).

Review URL: https://codereview.chromium.org/424343003

git-svn-id: svn://svn.chromium.org/blink/trunk@179242 bbb929c8-8fbe-4397-9dbb-9b2b20218538