Inline BitmpaImage::isAllDataReceived

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

Review URL: https://codereview.chromium.org/503623002

git-svn-id: svn://svn.chromium.org/blink/trunk@180832 bbb929c8-8fbe-4397-9dbb-9b2b20218538