Allocate memory in SkTDynamicHash on first use.

Allocate memory in SkTDynamicHash on first use.

This eliminates any dynamic allocation for hash tables that are never used.
This helps SkPicture, where some tables (SkPaint) are almost always used, but
some rarely (SkMatrix) or never (SkRegion).

This also removes the (as yet unimportant) ability for the hash table to
shrink.  This makes resizing harder to reason about, so I'd like to leave it
out until we see a need.

BUG=skia:1850
R=tomhudson@chromium.org, reed@google.com

Author: mtklein@google.com

Review URL: https://codereview.chromium.org/136403004

git-svn-id: http://skia.googlecode.com/svn/trunk@13051 2bbb7eff-a529-9590-31e7-b0007b416f81