Split up ios/web:test_support.

No test files are in ios/web, so this creates BUILD.gn files
for the subdirectories. Update clients outside of ios/web to
depend on ios/web/public/test/*, instead of ios/web/test.

BUG=684616
TBR=msarda,droger

Review-Url: https://codereview.chromium.org/2898733003
Cr-Commit-Position: refs/heads/master@{#475962}
159 files changed