Add fromCSSValue methods to LengthValue subclasses, remove extra method in CSSSimpleLength.

Does not implement CSSCalcLength::fromCSSValue yet, as that will
follow up in a larger change.

Also moved some methods in CSSSimpleLength from the header file
to the cpp file for tidiness.

BUG=545318

Review-Url: https://codereview.chromium.org/2373233002
Cr-Commit-Position: refs/heads/master@{#421771}
8 files changed