[sksg] Switch containers from SkT(D)Array to std::vector

Skia's containers grow too aggressively, stick to STL containers for
reduced head overhead.

Also introduce Group::shrink_to_fit() to be used when clients know they
are done adding children to a group.

TBR=
Bug: skia:8340
Change-Id: I842db2e80c7cfb65a462e4859af6e63496863a5c
Reviewed-on: https://skia-review.googlesource.com/151161
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
6 files changed