Improve web test documentation

- Combine rebaseline documentation in web_tests.md into
  web_test_expectations.md

- Format and add more details in #Testing-Runtime-Flags in
  web_tests.md, and add a link to it from web_test_expectationd.md.

- Prefer --flag-specific in some cases.

- Add rebaseline script as a rebaseline method.

- Mention the glob syntax of test expectations, and add a link to the
  more complete and up-to-date documentation of the syntax.

Bug: 1234863
Change-Id: Ia8a1dd140db3855fce391974a64ac742c6f8dfc1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3064798


Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Weizhong Xia <weizhong@google.com>
Cr-Commit-Position: refs/heads/master@{#907385}