C++ migration: add a C++ implementation of `FSTExponentialBackoff` (#1465)

This is a pretty close port of `FSTExponentialBackoff`. The changes are pretty minor:

* delay is calculated using <chrono> duration types, not plain numbers, which should be a little more type-safe;
* split a piece of code into a ClampDelay function, because it's reasonably close to std::clamp;
* rephrased the class-level comment to make it clearer that the first attempt always has delay = 0;
* added simple tests (other platforms don't have tests for this).

Also make sure that canceling a DelayedOperation is always valid.

1 files changed, 3 insertions, 1 deletions

diff --git a/Firestore/core/src/firebase/firestore/util/executor.h b/Firestore/core/src/firebase/firestore/util/executor.h
index df8b0b5..ea67b17 100644
--- a/Firestore/core/src/firebase/firestore/util/executor.h
+++ b/Firestore/core/src/firebase/firestore/util/executor.h
@@ -38,7 +38,9 @@ class DelayedOperation {
   // If the operation has not been run yet, cancels the operation. Otherwise,
   // this function is a no-op.
   void Cancel() {
-    cancel_func_();
+    if (cancel_func_) {
+      cancel_func_();
+    }
   }
 
   // Internal use only.