Fix leaked array buffer allocators of isolates

The array buffer allocators are allocated and owned by pdfium code,
they should be deleted properly after the corresponding isolates are
disposed.

BUG=pdfium:242

Review-Url: https://codereview.chromium.org/2254123004
diff --git a/fxjs/cfxjse_isolatetracker.cpp b/fxjs/cfxjse_isolatetracker.cpp
index 9594df3..adf6f2a 100644
--- a/fxjs/cfxjse_isolatetracker.cpp
+++ b/fxjs/cfxjse_isolatetracker.cpp
@@ -12,8 +12,11 @@
 
 CFXJSE_IsolateTracker::~CFXJSE_IsolateTracker() {}
 
-void CFXJSE_IsolateTracker::Append(v8::Isolate* pIsolate) {
+void CFXJSE_IsolateTracker::Append(
+    v8::Isolate* pIsolate,
+    std::unique_ptr<v8::ArrayBuffer::Allocator> alloc) {
   m_OwnedIsolates.push_back(pIsolate);
+  m_AllocatorMap[pIsolate] = std::move(alloc);
 }
 
 void CFXJSE_IsolateTracker::Remove(
@@ -24,6 +27,8 @@
   if (bFound)
     m_OwnedIsolates.erase(it);
   lpfnDisposeCallback(pIsolate, bFound);
+
+  m_AllocatorMap.erase(pIsolate);
 }
 
 void CFXJSE_IsolateTracker::RemoveAll(
@@ -32,4 +37,5 @@
     lpfnDisposeCallback(pIsolate, true);
 
   m_OwnedIsolates.clear();
+  m_AllocatorMap.clear();
 }