Use old ArrayBuffer free by default

If the new Free function is not implemented, but we are freeing a Normal
allocation, as opposed to one with guard regions, we can fall back on the
existing Free function.

Because guard regions are not yet used in normal circumstances, this will let
embedders who have not implemented the improve ArrayBuffer::Allocator interface
to continue working.

Bug: 
Change-Id: I2e30b523ef7493ab288110b90d8f994bfcfbc9b7
Reviewed-on: https://chromium-review.googlesource.com/508897
Commit-Queue: Eric Holk <eholk@chromium.org>
Commit-Queue: Brad Nelson <bradnelson@chromium.org>
Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45408}

src/api.cc

@@ -445,7 +445,16 @@ void* v8::ArrayBuffer::Allocator::Reserve(size_t length) { UNIMPLEMENTED(); }
 
 void v8::ArrayBuffer::Allocator::Free(void* data, size_t length,
                                       AllocationMode mode) {
-  UNIMPLEMENTED();
+  switch (mode) {
+    case AllocationMode::kNormal: {
+      Free(data, length);
+      return;
+    }
+    case AllocationMode::kReservation: {
+      UNIMPLEMENTED();
+      return;
+    }
+  }
 }
 
 void v8::ArrayBuffer::Allocator::SetProtection(