base: Rename discardable memory allocator interface and remove unnecessary class.

There's only one discardable memory allocator interface. Rename it
to DiscardableMemoryAllocator. DiscardableMemoryShmemChunk class
is unnecessary, remove it.

BUG=442945,422953

Review URL: https://codereview.chromium.org/1001873002

Cr-Commit-Position: refs/heads/master@{#321008}
diff --git a/ash/test/test_suite.cc b/ash/test/test_suite.cc
index 80d61f4..4f1ead0 100644
--- a/ash/test/test_suite.cc
+++ b/ash/test/test_suite.cc
@@ -53,8 +53,7 @@
   ui::ResourceBundle::InitSharedInstanceWithLocale(
       "en-US", NULL, ui::ResourceBundle::LOAD_COMMON_RESOURCES);
 
-  base::DiscardableMemoryShmemAllocator::SetInstance(
-      &discardable_memory_allocator_);
+  base::DiscardableMemoryAllocator::SetInstance(&discardable_memory_allocator_);
 }
 
 void AuraShellTestSuite::Shutdown() {