[d8] Override memory functions for PredictablePlatform.

- Implements GetPageAllocator and OnCriticalMemoryPressure methods, delegating
  to the default platform, in preparation for switching over to these for
  handling memory management.

Bug: chromium:756050
Change-Id: I66edb35ce2669ee9ffd53e82a63b598cc5b7f40c
Reviewed-on: https://chromium-review.googlesource.com/845035
Commit-Queue: Bill Budge <bbudge@chromium.org>
Reviewed-by: Eric Holk <eholk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50330}

src/d8.cc

@@ -197,6 +197,18 @@ class PredictablePlatform : public Platform {
     DCHECK_NOT_NULL(platform_);
   }
 
+  PageAllocator* GetPageAllocator() override {
+    return platform_->GetPageAllocator();
+  }
+
+  void OnCriticalMemoryPressure() override {
+    platform_->OnCriticalMemoryPressure();
+  }
+
+  bool OnCriticalMemoryPressure(size_t length) override {
+    return platform_->OnCriticalMemoryPressure(length);
+  }
+
   std::shared_ptr<TaskRunner> GetForegroundTaskRunner(
       v8::Isolate* isolate) override {
     return platform_->GetForegroundTaskRunner(isolate);