Add method to flatten CSSPrimitiveValues and CSSCalcValues by length units

This patch introduces CSSLengthArray as a flat representation
of a CSS length expression.
Example: calc(1in + (20% - 12px) / 2) is flattened down to 90px and 10%.
Methods are added to CSSPrimitiveValue and CSSCalcValue to constuct
the flattened representation of themselves.

Review URL: https://codereview.chromium.org/227693011

git-svn-id: svn://svn.chromium.org/blink/trunk@171150 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Fix flaky test fast/canvas/2d.text.draw.fill.maxWidth.gradient.html

Replacing setTimeout with font load event listener

BUG=245644

Review URL: https://codereview.chromium.org/224733020

git-svn-id: svn://svn.chromium.org/blink/trunk@171151 bbb929c8-8fbe-4397-9dbb-9b2b20218538