The majority of render tasks have 0, 1 or 2 dependencies, except for pictures that typically have dozens to hundreds of dependencies. SmallVec with 2 inline elements avoids many tiny heap allocations in pages with a lot of text shadows and other types of render tasks.

Differential Revision: https://phabricator.services.mozilla.com/D60151

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/db0378df4c6616330779c46cd830dd2f76f53b1b