[//components/safe_browsing] Consolidate ios/ subdirectory

Now that we eliminated problems in the internal dependency structure of
//components/safe_browsing, we can coalesce the directory layout. This
CL does so for //components/safe_browsing/ios. All of this code is
browser-process code (essentially by definition on iOS), so we
consolidate it into //components/safe_browsing/ios/browser.

Bug: 1216341
Change-Id: I1e0f1865130795d2012f60f6261795c2b6f7eb47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2975772
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Xinghui Lu <xinghuilu@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#895506}