Unstick background position origins

In the FillLayer class, reset the {x,y} origins each time {x,y}
position is set and require the caller to later call
setBackground{X,Y}Origin to override the default value. This fits
better with the way 'initial' values are set from the StyleBuilder and
avoids the problem of origins lingering in the FillLayer when setting
the 2-value versions of background-position.

Furthermore, split the flag m_backgroundOriginSet into one for each
property so that something like 'background-position-x:initial' won't
reset the position origin for both properties.

Also, remove code for handling 'initial' for {x,y} position, 'initial'
is already handled in the StyleBuilder further up the call chain.

BUG=304721

Review URL: https://codereview.chromium.org/35303002

git-svn-id: svn://svn.chromium.org/blink/trunk@160243 bbb929c8-8fbe-4397-9dbb-9b2b20218538