Implement SkAshmemDiscardableMemory

-   Implement ashmem-backed SkDiscardableMemory subclass:
    This class in only accesible via the SkDiscardableMemory::Create()
    function, which replaces the mock implementation in
    SkDiscardableMemory_none.cpp

-   Added SkDiscardableMemory_ashmem.cpp to the Android port of Skia
    Removed SkDiscardableMemory_none.cpp from the Android port.

-   Added DiscardableMemoryTest.
    Still needs work.

-   SkDiscardablePixelRef Bugfix:
    onLockPixels() now calls SkDELETE on the SkDiscardableMemory pointer
    when it fails to unlock.

-   Improved documentation inside ashmem.h

BUG=
R=scroggo@google.com

Review URL: https://codereview.chromium.org/83563002

git-svn-id: http://skia.googlecode.com/svn/trunk@12608 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/tests/CachedDecodingPixelRefTest.cpp b/tests/CachedDecodingPixelRefTest.cpp
index 6abcf3d..1e4ab3f 100644
--- a/tests/CachedDecodingPixelRefTest.cpp
+++ b/tests/CachedDecodingPixelRefTest.cpp
@@ -300,12 +300,12 @@
     REPORTER_ASSERT(reporter, 0 == pool->getRAMUsed());
 
     SkDiscardableMemoryPool* globalPool = SkGetGlobalDiscardableMemoryPool();
+    // Only acts differently from NULL on a platform that has a
+    // default discardable memory implementation that differs from the
+    // global DM pool.
     CheckPixelRef(TestImageGenerator::kFailGetPixels_TestType,
                   reporter, kSkDiscardable_PixelRefType, globalPool);
     CheckPixelRef(TestImageGenerator::kSucceedGetPixels_TestType,
                   reporter, kSkDiscardable_PixelRefType, globalPool);
-
-    // TODO(halcanary): When ashmem-backed SkDiscardableMemory lands,
-    // test that here (on platforms where it is availible).
 }
 ////////////////////////////////////////////////////////////////////////////////