Prefer using RenderSVGResourceMode when possible

It's not too difficult to keep track of when we're about to paint text,
so all methods in SVGInlineTextBoxPainter taking
RenderSVGResourceModeFlags can be adjusted to take RenderSVGResourceMode
instead, and then ApplyToTextMode can be ored in to the few (one) calls
where it's needed.
With the use (setting) of the ApplyToTextMode flag limited to a single
method, it makes sense to sink the calls to setTextDrawingMode out of
RenderSVGResource::updateGraphicsContext.
Bonus: Somewhat increased type-safety.

BUG=420022

Review URL: https://codereview.chromium.org/637503003

git-svn-id: svn://svn.chromium.org/blink/trunk@183398 bbb929c8-8fbe-4397-9dbb-9b2b20218538