Add BitmapImage isAllDataReceived() method

Add a bitmap image method that reports if all the encoded image
data has been received. No change in behavior, so no new layout
tests, add unit tests.

TEST=webkit_unit_test --gtest_filter="BitmapImage*DataReceived"
BUG=369901

Review URL: https://codereview.chromium.org/265093007

git-svn-id: svn://svn.chromium.org/blink/trunk@173381 bbb929c8-8fbe-4397-9dbb-9b2b20218538