[FileAPI] Don't start FileReaders on a context that is destroyed.

The FileReaders themselves will abort once they realize the context is
destroyed anyway, so it is pretty pointless (and potentially
problematic) to start new FileReaders after the context is destroyed.

Bug: 836724
Change-Id: I7bcbdf7d7905448df794df423a3fb0fe1697dd95
Reviewed-on: https://chromium-review.googlesource.com/1028555
Commit-Queue: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553726}

third_party/blink/renderer/core/fileapi/file_reader.cc

@@ -166,6 +166,10 @@ class FileReader::ThrottlingController final
   }
 
   void ExecuteReaders() {
+    // Dont execute more readers if the context is already destroyed (or in the
+    // process of being destroyed).
+    if (GetSupplementable()->IsContextDestroyed())
+      return;
     while (running_readers_.size() < max_running_readers_) {
       if (pending_readers_.IsEmpty())
         return;