Add a DiscardableMemoryShmemeAllocator.

mojo/services/html_viewer/html_viewer.cc

Index: mojo/services/html_viewer/html_viewer.cc
diff --git a/mojo/services/html_viewer/html_viewer.cc b/mojo/services/html_viewer/html_viewer.cc
index 8c71a9f3ab55ab08188141b4208174ff49dc615b..32621ac3c212f4f95dcc36de5fb7488629577c25 100644
--- a/mojo/services/html_viewer/html_viewer.cc
+++ b/mojo/services/html_viewer/html_viewer.cc
@@ -10,6 +10,7 @@
 #include "base/threading/thread.h"
 #include "gin/public/isolate_holder.h"
 #include "mojo/application/application_runner_chromium.h"
+#include "mojo/services/html_viewer/discardable_memory_allocator.h"
 #include "mojo/services/html_viewer/html_document.h"
 #include "mojo/services/html_viewer/mojo_blink_platform_impl.h"
 #include "mojo/services/html_viewer/webmediaplayer_factory.h"
@@ -59,6 +60,9 @@ const char kDisableEncryptedMedia[] = "disable-encrypted-media";
 // Prevents creation of any output surface.
 const char kIsHeadless[] = "is-headless";
 
+// Only have 20 megabytes of discardable memory to start out with.

[jamesr, 2015/03/18 22:40:14]

what do you mean "to start out with"? it doesn't seem that this value can change
at runtime. if this is intended as a statement about the history of the code,
put it in the patch description

+size_t kMaxMemory = 20 * 1024 * 1024;

[jamesr, 2015/03/18 22:40:14]

if i'm reading the allocator correctly the first allocation of any amount of
discardable memory will succeed (i.e. allocating 50MB would work) so this isn't
really a max value, it's a value that the system will try to evict down towards
on future allocations. maybe call this a target or something if that's the
intended behavior?

[Elliot Glaysher, 2015/03/18 23:54:44]

Renamed to 'kDesiredMaxMemory', and moved the above comment into the commit
message.

+
 class HTMLViewer;
 
 class HTMLViewerApplication : public mojo::Application {
@@ -161,13 +165,18 @@ class ContentHandlerImpl : public mojo::InterfaceImpl<ContentHandler> {
 class HTMLViewer : public mojo::ApplicationDelegate,
                    public mojo::InterfaceFactory<ContentHandler> {
  public:
-  HTMLViewer() : compositor_thread_("compositor thread") {}
+  HTMLViewer()
+      : discardable_memory_allocator_(kMaxMemory),
+        compositor_thread_("compositor thread") {}
 
   ~HTMLViewer() override { blink::shutdown(); }
 
  private:
   // Overridden from ApplicationDelegate:
   void Initialize(mojo::ApplicationImpl* app) override {
+    base::DiscardableMemoryAllocator::SetInstance(
+        &discardable_memory_allocator_);
+
     blink_platform_.reset(new MojoBlinkPlatformImpl(app));
 #if defined(V8_USE_EXTERNAL_STARTUP_DATA)
     // Note: this requires file system access.
@@ -234,6 +243,14 @@ class HTMLViewer : public mojo::ApplicationDelegate,
         &request);
   }
 
+  // Skia requires that we have one of these. Unlike the one used in chrome,
+  // this doesn't use purgable shared memory. Instead, it tries to free the
+  // oldest unlocked chunks on allocation.
+  //
+  // TODO(erg): In the long run, delete this allocator and get the real shared
+  // memory based purging allocator working here.
+  DiscardableMemoryAllocator discardable_memory_allocator_;
+
   scoped_ptr<MojoBlinkPlatformImpl> blink_platform_;
   base::Thread compositor_thread_;
   scoped_ptr<WebMediaPlayerFactory> web_media_player_factory_;