FontFaceSet.load() and FontFace.load() should not make temporary FontData

These methods kicked off font load by creating temporary FontData objects
and calling beginLoadIfNeeded() of them, while CSSFontFace::willUseFontData()
directly uses CSSFontFace.beginLoadIfNeeded(). This patch merges those code
paths. The new method CSSFontFace::load() kicks off font load, and set the
load status accordingly.

TEST=no new test, covered by existing tests
BUG=53213

Review URL: https://codereview.chromium.org/145403003

git-svn-id: svn://svn.chromium.org/blink/trunk@165658 bbb929c8-8fbe-4397-9dbb-9b2b20218538