Add SkImageGenerator Interface

-   Add SkDiscardablePixelRef class that uses SkDiscardableMemory and
    a SkImageGenerator.

-   Add SkDecodingImageGenerator class as an example of a
    SkImageGenerator.

-   Add DecodingImageGenerator unit test.

-   Add SkBasicDiscardableMemory implmentation for unit tests only.

R=reed@google.com, scroggo@google.com

Review URL: https://codereview.chromium.org/74793011

git-svn-id: http://skia.googlecode.com/svn/trunk@12341 2bbb7eff-a529-9590-31e7-b0007b416f81
10 files changed