[platform] Add NumberOfAvailableBackgroundThreads

The call can be used by the embedder to provide information on the workers
executing background tasks.

BUG=chromium:524425
LOG=N

Review URL: https://codereview.chromium.org/1664203004

Cr-Commit-Position: refs/heads/master@{#33788}

include/v8-platform.h

@@ -5,6 +5,7 @@
 #ifndef V8_V8_PLATFORM_H_
 #define V8_V8_PLATFORM_H_
 
+#include <stddef.h>
 #include <stdint.h>
 
 namespace v8 {
@@ -55,6 +56,15 @@ class Platform {
 
   virtual ~Platform() {}
 
+  /**
+   * Gets the number of threads that are used to execute background tasks. Is
+   * used to estimate the number of tasks a work package should be split into.
+   * A return value of 0 means that there are no background threads available.
+   * Note that a value of 0 won't prohibit V8 from posting tasks using
+   * |CallOnBackgroundThread|.
+   */
+  virtual size_t NumberOfAvailableBackgroundThreads() { return 0; }
+
   /**
    * Schedules a task to be invoked on a background thread. |expected_runtime|
    * indicates that the task will run a long time. The Platform implementation

src/libplatform/default-platform.cc

@@ -193,5 +193,10 @@ const char* DefaultPlatform::GetCategoryGroupName(
   static const char dummy[] = "dummy";
   return dummy;
 }
+
+size_t DefaultPlatform::NumberOfAvailableBackgroundThreads() {
+  return static_cast<size_t>(thread_pool_size_);
+}
+
 }  // namespace platform
 }  // namespace v8

src/libplatform/default-platform.h

@@ -34,6 +34,7 @@ class DefaultPlatform : public Platform {
   bool PumpMessageLoop(v8::Isolate* isolate);
 
   // v8::Platform implementation.
+  size_t NumberOfAvailableBackgroundThreads() override;
   void CallOnBackgroundThread(Task* task,
                               ExpectedRuntime expected_runtime) override;
   void CallOnForegroundThread(v8::Isolate* isolate, Task* task) override;