[ios] add dynamic types to qr code label

Bug: 891291
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I8e72497ea3a71a2cb2c6b8e51e4b5076b233457c
Reviewed-on: https://chromium-review.googlesource.com/c/1264538Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: David Jean <djean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597495}

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

@@ -474,6 +474,8 @@ CGFloat GetViewportSize() {
   UILabel* viewportCaption = [[UILabel alloc] init];
   NSString* label = l10n_util::GetNSString(IDS_IOS_QR_SCANNER_VIEWPORT_CAPTION);
   [viewportCaption setText:label];
+  [viewportCaption
+      setFont:[UIFont preferredFontForTextStyle:UIFontTextStyleBody]];
   [viewportCaption setNumberOfLines:0];
   [viewportCaption setTextAlignment:NSTextAlignmentCenter];
   [viewportCaption setAccessibilityLabel:label];