base: Add uncached malloc based discardable memory type.

base/memory/discardable_memory_unittest.cc

Index: base/memory/discardable_memory_unittest.cc
diff --git a/base/memory/discardable_memory_unittest.cc b/base/memory/discardable_memory_unittest.cc
index d4a230a46fa7c3d2257d17aa156cc90b226ba7cc..d4c295565508171dc471fbf07df535609523275e 100644
--- a/base/memory/discardable_memory_unittest.cc
+++ b/base/memory/discardable_memory_unittest.cc
@@ -77,9 +77,8 @@ TEST_P(DiscardableMemoryTest, LockAndUnLock) {
   ASSERT_NE(static_cast<void*>(NULL), addr);
 
   memory->Unlock();
-  // The system should have no reason to purge discardable blocks in this brief
-  // interval, though technically speaking this might flake.
-  EXPECT_EQ(DISCARDABLE_MEMORY_LOCK_STATUS_SUCCESS, memory->Lock());
+
+  EXPECT_NE(DISCARDABLE_MEMORY_LOCK_STATUS_FAILED, memory->Lock());
   addr = memory->Memory();
   ASSERT_NE(static_cast<void*>(NULL), addr);