Add functions for inspecting SkShader.

Add functions for inspecting SkShader.

Add a function to inspect an SkShader to determine if it is an
SkComposeShader.

Add a virtual function for determining if an SkShader is a custom
shader, which returns a custom set of information. The
implementation is in Android, and this function is only defined
if SK_BUILD_FOR_ANDROID_FRAMEWORK.

BUG=b/10650594
R=reed@google.com, scroggo@google.com

Author: djsollen@google.com

Review URL: https://codereview.chromium.org/282733005

git-svn-id: http://skia.googlecode.com/svn/trunk@14716 2bbb7eff-a529-9590-31e7-b0007b416f81