Attempt to make it more clear what FloatIntervalSearchAdaptor::collectIfNeeded is doing.

It seemed to me that template specifications would be clearer
than an if.  They also allow for compile-time error checking
were a 3rd type of float to come into existance in CSS4. :p

For any unfamiliar with this method, this the object
used for performing a search on a RedBlackTree in WTF.

We create one of these adaptors, specifying that we want to
search for values in a specific (logical) Y interval, and
this adaptor is called back for any values in the RBTree
cooresponding to that interval range.

The job of this adaptor is to collect the various values
we care about, including the left or right-most offset
of the floats in that Y-range as well as what the last
(document order) float seen in that range.

It also collects the remaining available height for the block but I'm less clear on how that parameter is used.

Review URL: https://chromiumcodereview.appspot.com/22493015

git-svn-id: svn://svn.chromium.org/blink/trunk@155885 bbb929c8-8fbe-4397-9dbb-9b2b20218538