LeakExpectations: mark svg fonts leaks as false-positives

HarfBuzzRunCache is a global cache with entries holding |Font| which indirectly hold strong ref to |SVGFontFace|.
In future, the leak detector should clear the cache before counting number of |Document|s.

These "leaks" disappear if following line is commented out:
WebKit/Source/platform/fonts/harfbuzz/HarfBuzzShaper.cpp
861         runCache.insert(key, new CachedShapingResults(harfBuzzBuffer.get(), m_font, props.direction, localeString));

BUG=366455
TBR=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/342153002

git-svn-id: svn://svn.chromium.org/blink/trunk@176490 bbb929c8-8fbe-4397-9dbb-9b2b20218538