Build SafeBrowsingUrlCheckerImpl on iOS

This builds SafeBrowsingUrlCheckerImpl on iOS as part of components_unittests.

The remaining content/ dependencies in SafeBrowsingUrlCheckerImpl are all
in methods related to real-time checks. This CL moves the implementations of
these content-dependent methods to a content-specific file, and defines stubs
for these methods on iOS, where real-time checks won't be initially supported.

Change-Id: I5f83e99b586ff4c62876248d8933b2671c1eceb8
Bug: 1028755
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2033332
Reviewed-by: Varun Khaneja <vakh@chromium.org>
Commit-Queue: Ali Juma <ajuma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737774}
6 files changed