skia: Added SkDiscardableMemory interface.

Chrome's implementation of SkDiscardableMemory is provided here:
https://codereview.chromium.org/23206002. SkDiscardableMemory is
intended to be used for image caching.

R=reed@google.com, scroggo@google.com, hclam@chromium.org, jamesr@chromium.org
BUG=229120

Author=ernstm@chromium.org

Submitted on behalf of ernstm@chromium.org due to commit queue bug.

Review URL: https://codereview.chromium.org/22950012

git-svn-id: http://skia.googlecode.com/svn/trunk@10797 2bbb7eff-a529-9590-31e7-b0007b416f81

gyp/ports.gyp

@@ -37,6 +37,7 @@
         '../src/ports/SkOSFile_posix.cpp',
         '../src/ports/SkOSFile_stdio.cpp',
         '../src/ports/SkOSFile_win.cpp',
+        '../src/ports/SkDiscardableMemory_none.cpp',
         '../src/ports/SkPurgeableMemoryBlock_none.cpp',
        #'../src/ports/SkThread_none.cpp',
         '../src/ports/SkThread_pthread.cpp',

src/core/SkDiscardableMemory.h

@@ -0,0 +1,54 @@
+/*
+ * Copyright 2013 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkDiscardableMemory_DEFINED
+#define SkDiscardableMemory_DEFINED
+
+#include "SkTypes.h"
+
+/**
+ *  Interface for discardable memory. Implementation is provided by the
+ *  embedder.
+ */
+class SkDiscardableMemory {
+
+public:
+    /**
+     *  Factory method that creates, initializes and locks an SkDiscardableMemory
+     *  object. If either of these steps fails, a NULL pointer will be returned.
+     */
+    static SkDiscardableMemory* Create(size_t bytes);
+
+    /** Must not be called while locked.
+     */
+    virtual ~SkDiscardableMemory() {}
+
+    /**
+     * Locks the memory, prevent it from being discarded. Once locked. you may
+     * obtain a pointer to that memory using the data() method.
+     *
+     * lock() may return false, indicating that the underlying memory was
+     * discarded and that the lock failed.
+     *
+     * Nested calls to lock are not allowed.
+     */
+    virtual bool lock() = 0;
+
+    /**
+     * Returns the current pointer for the discardable memory. This call is ONLY
+     * valid when the discardable memory object is locked.
+     */
+    virtual void* data() = 0;
+
+    /**
+     * Unlock the memory so that it can be purged by the system. Must be called
+     * after every successful lock call.
+     */
+    virtual void unlock() = 0;
+};
+
+#endif

src/ports/SkDiscardableMemory_none.cpp

@@ -0,0 +1,12 @@
+/*
+ * Copyright 2013 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkDiscardableMemory.h"
+
+SkDiscardableMemory* SkDiscardableMemory::Create(size_t bytes) {
+    return NULL;
+}