Call DiscardableMemoryAllocator::SetInstance()

Apparently this can cause a crash when when we try to render an emoji at a
different size than the emoji font's native bitmap size. I'm not sure how to
reproduce the issue. I'll ask Hixie when he gets back.

Currently we just use regular memory. In the future, we should actually use
discarable memory.

Fixes #203

sky/shell/shell.cc

@@ -6,6 +6,9 @@
 
 #include "base/bind.h"
 #include "base/i18n/icu_util.h"
+#include "base/lazy_instance.h"
+#include "base/memory/discardable_memory.h"
+#include "base/memory/discardable_memory_allocator.h"
 #include "base/single_thread_task_runner.h"
 #include "mojo/common/message_pump_mojo.h"
 #include "mojo/edk/embedder/embedder.h"
@@ -23,6 +26,27 @@ scoped_ptr<base::MessagePump> CreateMessagePumpMojo() {
   return make_scoped_ptr(new mojo::common::MessagePumpMojo);
 }
 
+class NonDiscardableMemory : public base::DiscardableMemory {
+ public:
+  explicit NonDiscardableMemory(size_t size) : data_(new uint8_t[size]) {}
+  bool Lock() override { return false; }
+  void Unlock() override {}
+  void* data() const override { return data_.get(); }
+
+ private:
+  scoped_ptr<uint8_t[]> data_;
+};
+
+class NonDiscardableMemoryAllocator : public base::DiscardableMemoryAllocator {
+ public:
+  scoped_ptr<base::DiscardableMemory> AllocateLockedDiscardableMemory(
+      size_t size) override {
+    return make_scoped_ptr(new NonDiscardableMemory(size));
+  }
+};
+
+base::LazyInstance<NonDiscardableMemoryAllocator> g_discardable;
+
 }  // namespace
 
 Shell::Shell(scoped_ptr<ServiceProviderContext> service_provider_context)
@@ -51,6 +75,8 @@ void Shell::Init(scoped_ptr<ServiceProviderContext> service_provider_context) {
 #if !defined(OS_LINUX)
   CHECK(gfx::GLSurface::InitializeOneOff());
 #endif
+  base::DiscardableMemoryAllocator::SetInstance(&g_discardable.Get());
+
   g_shell = new Shell(service_provider_context.Pass());
 }