change backing store cache to be memory-based rather than count

chrome/browser/renderer_host/backing_store_mac.cc

Index: chrome/browser/renderer_host/backing_store_mac.cc
diff --git a/chrome/browser/renderer_host/backing_store_mac.cc b/chrome/browser/renderer_host/backing_store_mac.cc
index 0381f24db7de3b39c7ab8a962f8716c39a7c64d8..d87b2927ba73b0ffe480d6dfb34d42789ecb5994 100644
--- a/chrome/browser/renderer_host/backing_store_mac.cc
+++ b/chrome/browser/renderer_host/backing_store_mac.cc
@@ -20,6 +20,11 @@ BackingStore::BackingStore(RenderWidgetHost* widget, const gfx::Size& size)
 BackingStore::~BackingStore() {
 }
 
+size_t BackingStore::MemorySize() {
+  // Always 4 bytes per pixel.
+  return size_.GetArea() * 4;
+}
+
 void BackingStore::PaintRect(base::ProcessHandle process,
                              TransportDIB* bitmap,
                              const gfx::Rect& bitmap_rect) {