[iOS] Put the QRCode scanner caption in scrollView

This CL puts the caption in a scroll view, allowing it to be scrolled
when the font size is big.

Bug: 916068
Change-Id: I111e1d4e00ea2949c85964999106248dfebb433c
Reviewed-on: https://chromium-review.googlesource.com/c/1392187
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619926}

ios/chrome/browser/ui/qr_scanner/BUILD.gn

@@ -27,6 +27,7 @@ source_set("qr_scanner") {
     "//ios/chrome/browser/ui/icons",
     "//ios/chrome/browser/ui/util",
     "//ios/chrome/common:ios_app_bundle_id_prefix_buildflags",
+    "//ios/chrome/common/ui_util",
     "//ios/third_party/material_components_ios",
     "//ui/base",
   ]