Create weblayer_browsertests target and one simple test.

This will be added to FYI bots in a followup cl.

Change-Id: I8ca2e3ce61e628cf6e342a8ab4fc604975c0d225
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1809897
Commit-Queue: John Abd-El-Malek <jam@chromium.org>
Auto-Submit: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697497}

• BUILD.gn[diff]
• weblayer/browser/browser_main_parts_impl.cc[diff]
• weblayer/browser/browser_main_parts_impl.h[diff]
• weblayer/shell/BUILD.gn[diff]
• weblayer/shell/app/shell_main.cc[diff]
• weblayer/shell/app/shell_main_params.cc[Added - diff]
• weblayer/shell/app/shell_main_params.h[Added - diff]
• weblayer/shell/browser/shell.h[diff]
• weblayer/shell/common/shell_switches.cc[Added - diff]
• weblayer/shell/common/shell_switches.h[Added - diff]
• weblayer/test/BUILD.gn[Added - diff]
• weblayer/test/DEPS[Added - diff]
• weblayer/test/browsertests_main.cc[Added - diff]
• weblayer/test/data/simple_page.html[Added - diff]
• weblayer/test/test_launcher_delegate_impl.cc[Added - diff]
• weblayer/test/test_launcher_delegate_impl.h[Added - diff]
• weblayer/test/weblayer_browser_test.cc[Added - diff]
• weblayer/test/weblayer_browser_test.h[Added - diff]
• weblayer/test/weblayer_browser_test_test.cc[Added - diff]
• weblayer/test/weblayer_browser_test_utils.cc[Added - diff]
• weblayer/test/weblayer_browser_test_utils.h[Added - diff]