Fix iOS canary compilation.

Currently compilation fails because there's an iOS 11 only constant
in a switch-case, so there's a warning -Wunguarded-availability-new.
This silences the warning when compiling this case.

Bug: 
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I3fe6bc7e66a16c3a8590af3472f345f8a883e452
Reviewed-on: https://chromium-review.googlesource.com/774858Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517101}

ios/chrome/browser/ui/qr_scanner/camera_controller.mm

@@ -369,9 +369,12 @@
         [self setCameraState:qr_scanner::MULTIPLE_FOREGROUND_APPS];
         break;
 #if defined(__IPHONE_11_1) && (__IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_11_1)
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wunguarded-availability-new"
       case AVCaptureSessionInterruptionReasonVideoDeviceNotAvailableDueToSystemPressure:
         [self setCameraState:qr_scanner::CAMERA_UNAVAILABLE_DUE_TO_SYSTEM_PRESSURE];
         break;
+#pragma clang diagnostic pop
 #endif
       case AVCaptureSessionInterruptionReasonAudioDeviceInUseByAnotherClient:
         NOTREACHED();