Make style building for 'font-size' less custom.

This change adds a FontDescription::Size structure which represents a
(specified) font size in transit. The conversion from CSSValue* to
font size (now FontDescription::Size) has been moved to
StyleBuilderConverter. This reduces the dependency on RenderStyle/CSSValue
in FontBuilder.

Keyword sizes are now resolved into actual float sizes in FontBuilder::
setSize. This happens only if the FontDescription::Size has a keyword size,
but has no float value size. The idea is that if both a keyword size
and float value size is provided from the outside, it means we inherit
the "entire" Size as-is, and no re-resolving of the keyword size is
necessary.

The m_fontSizehasViewportUnits flag has been removed. StyleBuilderConverter
is now marking the relevant style directly (more or less) during conversion
instead.

Review URL: https://codereview.chromium.org/468793003

git-svn-id: svn://svn.chromium.org/blink/trunk@182154 bbb929c8-8fbe-4397-9dbb-9b2b20218538