Introduce new API for detecting when the embedder is in foreground/background.

This will allow us to remove background idle notification calls in Chrome.

BUG=chromium:490559
LOG=NO

Review URL: https://codereview.chromium.org/1406303008

Cr-Commit-Position: refs/heads/master@{#31464}

include/v8.h

@@ -5806,6 +5806,18 @@ class V8_EXPORT Isolate {
    */
   int ContextDisposedNotification(bool dependant_context = true);
 
+  /**
+   * Optional notification that the isolate switched to the foreground.
+   * V8 uses these notifications to guide heuristics.
+   */
+  void IsolateInForegroundNotification();
+
+  /**
+   * Optional notification that the isolate switched to the background.
+   * V8 uses these notifications to guide heuristics.
+   */
+  void IsolateInBackgroundNotification();
+
   /**
    * Allows the host application to provide the address of a function that is
    * notified each time code is added, moved or removed.

src/api.cc

@@ -7379,6 +7379,18 @@ int Isolate::ContextDisposedNotification(bool dependant_context) {
 }
 
 
+void Isolate::IsolateInForegroundNotification() {
+  i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);
+  return isolate->heap()->SetOptimizeForLatency();
+}
+
+
+void Isolate::IsolateInBackgroundNotification() {
+  i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);
+  return isolate->heap()->SetOptimizeForMemoryUsage();
+}
+
+
 void Isolate::SetJitCodeEventHandler(JitCodeEventOptions options,
                                      JitCodeEventHandler event_handler) {
   i::Isolate* isolate = reinterpret_cast<i::Isolate*>(this);

src/heap/heap.h

@@ -1029,6 +1029,8 @@ class Heap {
   bool HasHighFragmentation();
   bool HasHighFragmentation(intptr_t used, intptr_t committed);
 
+  void SetOptimizeForLatency() { optimize_for_memory_usage_ = false; }
+  void SetOptimizeForMemoryUsage() { optimize_for_memory_usage_ = true; }
   bool ShouldOptimizeForMemoryUsage() { return optimize_for_memory_usage_; }
 
   // ===========================================================================