Merge FixedIntegerConversion and FixedFloatConversion for CSSPrimitiveValue::convertToLength

Continuing from crrev.com/71253002, the two Fixed conversion options
in convertToLength behave almost identically, save for some clamping
differences. The FixedIntegerConversion clamps to about the range of
LayoutUnits (+-32mil) and doesn't actually convert to an integer.
FixedFloatConversion simple doesn't clamp.

Almost all Length conversions use the clamped path and it doesn't
seem like the differences are intentional. The non-clamping paths
are calcs, blurs in filters, and lengths in transforms. I've made
all the conversions clamp for consistency in getComputedStyle (as
otherwise properties which return used values will be clamped and
those which return computed values won't).

For reference the clamping behaviour was introduced in wkb.ug/102735

BUG=

Review URL: https://codereview.chromium.org/68203023

git-svn-id: svn://svn.chromium.org/blink/trunk@162038 bbb929c8-8fbe-4397-9dbb-9b2b20218538