Const-correct DelayedEntryCompare.

libc++'s _LIBCPP_DEBUG gets upset when comparators take non-const
references.

Bug: chromium:893810
Change-Id: I838ff08bfd53893984f0ce41a9d78d6f1d80a324
Reviewed-on: https://chromium-review.googlesource.com/c/1272184Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: David Benjamin <davidben@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56533}

src/libplatform/default-foreground-task-runner.h

@@ -68,7 +68,7 @@ class V8_PLATFORM_EXPORT DefaultForegroundTaskRunner
   // queue. This is necessary because we have to reset the unique_ptr when we
   // remove a DelayedEntry from the priority queue.
   struct DelayedEntryCompare {
-    bool operator()(DelayedEntry& left, DelayedEntry& right) {
+    bool operator()(const DelayedEntry& left, const DelayedEntry& right) const {
       return left.first > right.first;
     }
   };