[safari-import] Only build `c/u/content:unit_tests` for non iOS

The `components/user_data_importer/content:unit_tests` target was being
built for non-blink builds, which usually means that is not build for
iOS. However, there is an experiment build for iOS that actually uses
blink. As such, a build failure due to duplicate symbol for
`MakeBookmarkParser` from both `IOSBookmarkParser` and
`ContentBookmarkParser`. See
https://ci.chromium.org/ui/p/chromium/builders/ci/ios-blink-rel-fyi/1624/overview

This CL moves this build target from being included for `use_blink` to
`!is_ios`.

Bug: 407587751, 414604427
Change-Id: I09f93ee38b7eff623e91144fccf6ebb79eb5b7db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6716079
Auto-Submit: Filipa Senra <fsenra@google.com>
Reviewed-by: Thomas Nguyen <tungnh@chromium.org>
Owners-Override: Thomas Nguyen <tungnh@chromium.org>
Commit-Queue: Filipa Senra <fsenra@google.com>
Cr-Commit-Position: refs/heads/main@{#1484229}