Unlike other types of render tasks, pictures can have hundreds of dependencies. The dependency vector is re-built every frame, leading to a lot of vector re-allocations in some pages.

Depends on D60151

Differential Revision: https://phabricator.services.mozilla.com/D60182

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/dfe4fa983d2b1eaaa3db2141a5e1c0f5ae58d5d7