Remembering file when saving.

When user clicks save button:
If user has opened image/draws on canvas (load image) for the first time,
save dialog box pops up. Thereafter, app saves to same file without
prompting user with dialog box.
Output errors:
Currently handled by saveFile function.

R=mgiuca@chromium.org

Review URL: https://codereview.appspot.com/38220043