Replace use of GC::alpha() in SVGImage::draw() with GC::getNormalizedAlpha()

As requested in https://codereview.chromium.org/181633002/

BUG=261177

Review URL: https://codereview.chromium.org/182853002

git-svn-id: svn://svn.chromium.org/blink/trunk@168049 bbb929c8-8fbe-4397-9dbb-9b2b20218538