[include] Make Platform::GetPageAllocator abstract

All embedders override this method now, so it can be abstract.

R=mlippautz@chromium.org

Bug: v8:12425
Change-Id: I4db5d74341c9519222592a88d247bc2aa2be03a9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3780538Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81910}

include/v8-platform.h

@@ -924,10 +924,7 @@ class Platform {
   /**
    * Allows the embedder to manage memory page allocations.
    */
-  virtual PageAllocator* GetPageAllocator() {
-    // TODO(bbudge) Make this abstract after all embedders implement this.
-    return nullptr;
-  }
+  virtual PageAllocator* GetPageAllocator() = 0;
 
   /**
    * Allows the embedder to specify a custom allocator used for zones.

test/unittests/compiler-dispatcher/compiler-dispatcher-unittest.cc

@@ -202,6 +202,8 @@ class MockPlatform : public v8::Platform {
   MockPlatform(const MockPlatform&) = delete;
   MockPlatform& operator=(const MockPlatform&) = delete;
 
+  PageAllocator* GetPageAllocator() override { UNIMPLEMENTED(); }
+
   int NumberOfWorkerThreads() override { return 1; }
 
   std::shared_ptr<TaskRunner> GetForegroundTaskRunner(