Move Length classes to platform and break core dependencies.

The classes related to Lengths are moved from core/platform and
core/css to platform.

The dependency on CSS was due to ViewportPercentage units needed by
LengthFunctions, and is broken by moving the floatValueForLength method
into platform while leaving the other methods from LengthFunctions.h in
a new file named CSSLengthFunctions. To resolve the abiguity between the
CSS and platform versions of floatValueForLength, the RenderView argument
is made compulsory on the CSS version.

Numerous files needed to be updated. Those that only use
floatValueForLength with no RenderView get the platform version,
while the others get the CSS version with a null RenderView if
necessary. Note that a null RenderView is legal - the previous code
supported it and the default value for the RenderView parameter was
null.

R=abarth@chromium.org, abarth, eseidel
TEST=None. Existing coverage is adequate.
BUG=297477

Review URL: https://codereview.chromium.org/27592002

git-svn-id: svn://svn.chromium.org/blink/trunk@159953 bbb929c8-8fbe-4397-9dbb-9b2b20218538