Stay in layer scroll if nothing to scroll

 Bug: 6628376
 The issue here is that contentX & contentY can both be 0 if we are scrolling
 slowly enough as they are adjusted by the page's scale and can thus round down.
 However, this would result in us falling out of layer drag mode even though
 we haven't tried to scroll past the edge of the layer. Detect this case, and
 stay in layer scroll mode.

Change-Id: I3c655d0d03e8f89887abbe718bd24699c133ee1a