Expand README for Blink's web_tests/webexposed directory

* Say more about the relationship to virtual/stable

* Say more about the different contexts tested, and call out the
  service worker and worklet tests that live elsewhere in the tree.

* Say more about the platform specific listings and there those
  results live.

* Call out caveat about inheritance changes.

Change-Id: Ifc750986f2b560ecc6c819bffd501b868a8d9da5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2278605
Commit-Queue: Joshua Bell <jsbell@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Jarryd Goodman <jarrydg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#787872}

third_party/blink/web_tests/webexposed/README.txt

-DumpRenderTree runs with a default set of runtime flags enabled.
-Thus the results in this directory serve to document Blink's
-complete API as visible during testing.
+The results in this directory serve to document Blink's complete API
+as "visible" (exposed to scripts) during testing. These tests are run
+with a default set of runtime flags enabled, including experimental
+features and test flags.
 
-virtual/stable/webexposed/ runs the same tests without the experimental and test
-flags enabled. It is testing what is exposed to the stable channel.
+web_tests/virtual/stable/webexposed/ runs the same tests without the
+experimental and test flags enabled. It is testing what is exposed to
+the stable channel. Changes there may require approval from blink API
+OWNERS.
+
+## Global Interface Listing Tests
+
+Additions to Blink IDL files with new interfaces, methods and
+attributes should result in changes to the expectations here. Such
+changes should normally be made behind experimental flags (e.g.
+[RuntimeEnabled=flag] in the IDL), and should not change the
+virtual/stable expectations. To learn more about runtime flags, see:
+third_party/blink/renderer/platform/RuntimeEnabledFeatures.md
+
+To learn more about Blink IDL files, start with:
+third_party/blink/renderer/bindings/README.md
+
+NOTE: Changes to the inheritance of an existing interface (e.g.
+changing an interface to derive from EventTarget) can't be guarded by
+runtime flags, and will introduce virtual/stable changes.
+
+The tests enumerate global interfaces, methods, getters/setters (how
+most IDL attributes are exposed) and attributes (other IDL
+attributes). They do not assert types or parameters. Individual
+features typically have idlharness.js-based tests that live in
+web_tests/external/wpt which more thoroughly exercise each API. See:
+https://web-platform-tests.org/writing-tests/idlharness.html
+
+
+The test files live in web_tests/webexposed:
+
+* global-interface-listing.html - Window context
+* global-interface-listing-dedicated-worker.html - Dedicated Worker context
+* global-interface-listing-shared-worker.html - Shared Worker context
+
+But also in web_tests/http/tests/serviceworker/webexposed:
+
+* global-interface-listing-service-worker.html - Service Worker context
+
+And also in web_tests/http/tests/worklet/webexposed:
+
+* global-interface-listing-paint-worklet.html - Worklet contexts
+
+Also in web_tests/web_exposed:
+
+* global-interface-listing-platform-specific.html - Tests the
+  interfaces listed in the platformSpecificInterfaces array in
+  web_tests/resources/global-interface-listing.js. The results appear
+  in:
+  * platform/win/virtual/stable/webexposed/global-interface-listing-platform-specific-expected.txt
+  * platform/mac/virtual/stable/webexposed/global-interface-listing-platform-specific-expected.txt
+  * platform/linux/virtual/stable/webexposed/global-interface-listing-platform-specific-expected.txt
+  * platform/win7/virtual/stable/webexposed/global-interface-listing-platform-specific-expected.txt