sensors: Always assume Sensor::InitSensorProxyIfNeeded() succeeds.

r762075 ("ExecutionContext post-migration cleanup - mdoules/ q-s"), part
of the changes that moved ExecutionContext from Document to
LocalDOMWindow, removed the condition in InitSensorProxyIfNeeded() that
would cause it to have a nullptr |sensor_proxy_|.

Remove the check in Sensor::Activate(), but add a DCHECK to avoid
regressions.

Change-Id: If29238fb7772599a3a4636e25535c51669d5da11
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2421448
Auto-Submit: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#809666}

third_party/blink/renderer/modules/sensor/sensor.cc

@@ -263,11 +263,7 @@ void Sensor::Activate() {
   DCHECK_EQ(state_, SensorState::kActivating);
 
   InitSensorProxyIfNeeded();
-  if (!sensor_proxy_) {
-    HandleError(DOMExceptionCode::kInvalidStateError,
-                "The Sensor is no longer associated to a frame.");
-    return;
-  }
+  DCHECK(sensor_proxy_);
 
   if (sensor_proxy_->IsInitialized())
     RequestAddConfiguration();