Webfonts should not block load event for more than one frame

This patch fixes a bug that frame's load event does not fire when
FontLoader::loadPendingFonts does not start any load because all the
pending fonts are already started loading. This happens when a webfont
is used in multiple iframes; only one of them actually starts the load.

After this patch, if a font is used in multiple frames, subsequent
frames do not schedule a load (hence do not block the load event).

BUG=401846
TEST=fast/css/font-face-iframe-onload.html

Review URL: https://codereview.chromium.org/457273002

git-svn-id: svn://svn.chromium.org/blink/trunk@180321 bbb929c8-8fbe-4397-9dbb-9b2b20218538