Fix EditingStyle::mergeStyle()'s handling of custom properties

      This patch fixes the logic of EditingStyle::mergeStyle() to correctly
handle custom properties. Currently it serializes the CSSValue and then
reparses it, which, aside from being inefficient, doesn't work for
custom properties as the custom property name is lost (since we only
have the enum value CSSPropertyVariable).

BUG=622420

Review-Url: https://codereview.chromium.org/2103043004
Cr-Commit-Position: refs/heads/master@{#410614}