don't combine single image bitmaps

don't combine single image bitmaps

Anchors and other links often contain multiple bitmaps, which must
be stitched together when analyzing the picture data to determine
if the anchor is fully visible, partially occluded, or fully hidden.
For instance, nine-patches in WebKit appear as adjacent bitmaps.

If a website design has two or more bitmaps adjacent to each other,
these may be mistakenly joined. By tracking whether the original
node contained exactly one img element, combining bitmaps based
on their location is avoided unncessarily.

bug:3373743
Change-Id: I5bcbdaec3097a2b839ee2444e512a0def6a016d0