Replace OS::NumberOfCores() with CPU::NumberOfProcessorsOnline().

src/cpu.h

Index: src/cpu.h
diff --git a/src/cpu.h b/src/cpu.h
index ef58b4cdd2f126f2fd4feeb8a822ba90aeb0a454..58c7f5d9d4a8b8610eaca8c00e7c8866c3d99931 100644
--- a/src/cpu.h
+++ b/src/cpu.h
@@ -99,6 +99,9 @@ class CPU V8_FINAL BASE_EMBEDDED {
   bool has_vfp3() const { return has_vfp3_; }
   bool has_vfp3_d32() const { return has_vfp3_d32_; }
 
+  // Returns the number of processors online.
+  static int NumberOfProcessorsOnline() V8_WARN_UNUSED_RESULT;
+
   // Initializes the cpu architecture support. Called once at VM startup.
   static void SetUp();