Refactor //ios:all target in preparation of upstreaming.

Add temporary empty targets for each of the targets that will exists
upstream one the upstreaming process is complete.

Add "all_tests" targets for each directory that will contains test
targets, and change //ios:all to depend on them instead of depending
on the individual test targets.

Move ios_chrome_unittests target definition from ios/chrome/BUILD.gn
to ios/chrome/test/BUILD.gn.

BUG=None

Review-Url: https://codereview.chromium.org/2570523002
Cr-Commit-Position: refs/heads/master@{#438789}

• ios/BUILD.gn[diff]
• ios/chrome/BUILD.gn[Deleted - diff]
• ios/chrome/app/BUILD.gn[diff]
• ios/chrome/test/BUILD.gn[diff]
• ios/chrome/test/earl_grey/BUILD.gn[Added - diff]
• ios/net/BUILD.gn[diff]
• ios/showcase/BUILD.gn[Added - diff]
• ios/showcase/test/BUILD.gn[Added - diff]
• ios/test/BUILD.gn[Added - diff]
• ios/testing/BUILD.gn[diff]
• ios/web/BUILD.gn[diff]
• ios/web/shell/test/BUILD.gn[diff]
• testing/buildbot/gn_isolate_map.pyl[diff]