Added missing weak ptr null check.

WeakLearningSessionImpl::PredictDistribution accessed the session,
without first checking if its weak ptr was valid.  This CL adds that
wp check.

Bug: 1159663
Change-Id: I2c26fd8cb22cd2c78d39267e58e8b79217cee1c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2602258Reviewed-by: Ted Meyer <tmathmeyer@chromium.org>
Commit-Queue: Frank Liberato <liberato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#839165}

media/learning/impl/learning_session_impl.cc

@@ -92,6 +92,8 @@ class WeakLearningTaskController : public LearningTaskController {
 
   void PredictDistribution(const FeatureVector& features,
                            PredictionCB callback) override {
+    if (!weak_session_)
+      return;
     controller_->AsyncCall(&LearningTaskController::PredictDistribution)
         .WithArgs(features, std::move(callback));
   }