Add DiscardableMemoryAllocator to work around FD limit issue.

base/memory/discardable_memory_android.cc

Index: base/memory/discardable_memory_android.cc
diff --git a/base/memory/discardable_memory_android.cc b/base/memory/discardable_memory_android.cc
index ffead39307e7a8c9a2a071a1179b30ed8e61b910..223168d4c992a77c804c257747f2e8d789805fad 100644
--- a/base/memory/discardable_memory_android.cc
+++ b/base/memory/discardable_memory_android.cc
@@ -33,7 +33,8 @@ const int kDiscardableMemoryNumLimit = 128;
 class DiscardableMemoryAndroid : public DiscardableMemory {
  public:
   DiscardableMemoryAndroid(int fd, void* address, size_t size)
-      : fd_(fd),
+      : DiscardableMemory(size),
+        fd_(fd),
         memory_(address),
         size_(size) {
     DCHECK_GE(fd_, 0);