Use BBH reserve hook to preallocate space for tiles.

Before getting too far into changing how SkTileGrid stores its tiles, I figured I'd
better see how much I can tweak out the existing format.  Cleverly, that way
any improvements I make by changing the format will look that much less
impressive.

This CL looks like it will be a 5-15% win in time spent recording, with no effect
on playback.

This CL also shrinks the tiles to fit exactly when we're done inserting,
using newly added SkTDArray::shrinkToFit().  It's quite cheap to run (maybe
taking back 1-2% from those 5-15% wins), and means we'll lug around about 15%
fewer bytes in the tile grids.  Note though this strategy temporarily uses up to
30% more memory while building the tile grid.  For our largest SKPs, that's
maybe 75-100K extra.

BUG=skia:

Review URL: https://codereview.chromium.org/639823005

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