Layout test svg/batik/text/smallFonts.svg is no longer flaky

svg/batik/text/smallFonts.svg has not been flaky on linux for over a
week now, was likely fixed by the font-size cache change.

BUG=250897
TBR=ojan@chromium.org

Review URL: https://codereview.chromium.org/32323007

git-svn-id: svn://svn.chromium.org/blink/trunk@160168 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Handle the font size specified in a canvas when is in Rem format.

The CSSPrimitive class was missing the documentElement to compute the font
size. Added test to check the situation.

BUG=303846

Review URL: https://codereview.chromium.org/32563002

git-svn-id: svn://svn.chromium.org/blink/trunk@160169 bbb929c8-8fbe-4397-9dbb-9b2b20218538