SK_ONCE for SkMatrix::I()

Going to start doing these in progressively larger and larger bulk,
but I figured the first few changes probably merit caution.

BUG=
R=reed@google.com, bungeman@google.com

Author: mtklein@google.com

Review URL: https://codereview.chromium.org/26905002

git-svn-id: http://skia.googlecode.com/svn/trunk@11721 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed