Make sure ImmutableStylePropertySet uses the new 16 bits size of StylePropertyMetadata.
    
We know that StylePropertyMetadata is 16 bits so we can use uint16_t as the type for the bitfields so that sizeof(StylePropertyMetadata) is equals to 2 bytes.
    
Cleanup two casts not necessary at the same time.
    
This should save up ~1.8Mb on Apple membuster test.

Review URL: https://chromiumcodereview.appspot.com/17448021

git-svn-id: svn://svn.chromium.org/blink/trunk@152910 bbb929c8-8fbe-4397-9dbb-9b2b20218538