Eliminate "FractionConversion" from CSSPrimitiveValue::convertToLength

The FractionConversion argument in CSSPrimitiveValue::convertToLength
doesn't seem to actually have a purpose. It is for converting numbers
into Length(x * 100, Percent), but this case shouldn't be reachable.
The original patch introducing it[1] doesn't shed any light either.

The only way I could find to get into this conversion was in the fast
path for parsing transforms, where e.g. translate(0) would get the 0
parsed as a number and not a <length>. This patch fixes this parser
error and removes the now unused FractionConversion argument. Note
that we can probably also merge the two Fixed conversions, as the
two only differ in clamping large numbers (probably unintended).

[1] https://bugs.webkit.org/show_bug.cgi?id=74913

BUG=

Review URL: https://codereview.chromium.org/71253002

git-svn-id: svn://svn.chromium.org/blink/trunk@161991 bbb929c8-8fbe-4397-9dbb-9b2b20218538