HostDiscardableSharedMemoryManager should store the default memory limit

OnMemoryStateChange() callback is invoked on the UI thread but we can't
call GetDefaultMemoryLimit() from the UI thread. Store the default limit
on construction.

BUG=652557

Review-Url: https://codereview.chromium.org/2388403002
Cr-Commit-Position: refs/heads/master@{#422700}

content/common/host_discardable_shared_memory_manager.cc

@@ -164,7 +164,8 @@ HostDiscardableSharedMemoryManager::MemorySegment::~MemorySegment() {
 }
 
 HostDiscardableSharedMemoryManager::HostDiscardableSharedMemoryManager()
-    : memory_limit_(GetDefaultMemoryLimit()),
+    : default_memory_limit_(GetDefaultMemoryLimit()),
+      memory_limit_(default_memory_limit_),
       bytes_allocated_(0),
       memory_pressure_listener_(new base::MemoryPressureListener(
           base::Bind(&HostDiscardableSharedMemoryManager::OnMemoryPressure,
@@ -352,7 +353,7 @@ void HostDiscardableSharedMemoryManager::OnMemoryStateChange(
     base::MemoryState state) {
   switch (state) {
     case base::MemoryState::NORMAL:
-      SetMemoryLimit(GetDefaultMemoryLimit());
+      SetMemoryLimit(default_memory_limit_);
       break;
     case base::MemoryState::THROTTLED:
       SetMemoryLimit(0);

content/common/host_discardable_shared_memory_manager.h

@@ -136,6 +136,7 @@ class CONTENT_EXPORT HostDiscardableSharedMemoryManager
   // a heap. The LRU memory segment always first.
   typedef std::vector<scoped_refptr<MemorySegment>> MemorySegmentVector;
   MemorySegmentVector segments_;
+  size_t default_memory_limit_;
   size_t memory_limit_;
   size_t bytes_allocated_;
   std::unique_ptr<base::MemoryPressureListener> memory_pressure_listener_;