[sensors] Remove the redundant security context check

As it follows from https://github.com/w3c/sensors/pull/315

Change-Id: I6e2adaae43153fdff3cb792f7e9741652f12f9c0
Reviewed-on: https://chromium-review.googlesource.com/725326Reviewed-by: Alexander Shalamov <alexander.shalamov@intel.com>
Commit-Queue: Mikhail Pozdnyakov <mikhail.pozdnyakov@intel.com>
Cr-Commit-Position: refs/heads/master@{#509747}

third_party/WebKit/Source/modules/sensor/Sensor.cpp

@@ -30,12 +30,8 @@ Sensor::Sensor(ExecutionContext* execution_context,
       type_(type),
       state_(SensorState::kIdle),
       last_reported_timestamp_(0.0) {
-  // Check secure context.
-  String error_message;
-  if (!execution_context->IsSecureContext(error_message)) {
-    exception_state.ThrowSecurityError(error_message);
-    return;
-  }
+  // [SecureContext] in idl.
+  DCHECK(execution_context->IsSecureContext());
 
   // Check top-level browsing context.
   if (!ToDocument(execution_context)->domWindow()->GetFrame() ||