Move CSSLengthFunctions contents into LengthFunctions

Since RenderViews are no longer required to resolve Lengths as of
crrev.com/82083002, we can merge the two files CSSLengthFunctions
and LengthFunctions.

I've also made some tiny clean-ups, removing the unused function
minimumIntValueForLength and the unneeded include dependency from
RenderStyle.h.

BUG=

Review URL: https://codereview.chromium.org/101763009

git-svn-id: svn://svn.chromium.org/blink/trunk@164588 bbb929c8-8fbe-4397-9dbb-9b2b20218538