Don't use Color::m_valid for indicating color parsing success

This patch changes the Color constructors taking String into a setter
setFromString, which returns whether the String was correctly parsed.
The motivation here is to stop using invalid colors for a whole variety
of purposes, with an end goal of removing the bool from Color and having
a separate class StyleColor with a bool m_isCurrentColor.

TBR=abarth@chromium.org
BUG=331126

Review URL: https://codereview.chromium.org/122713002

git-svn-id: svn://svn.chromium.org/blink/trunk@164441 bbb929c8-8fbe-4397-9dbb-9b2b20218538