Add detailed how-to-roll in web_tests/webgpu readme

Bug: None
Change-Id: Icc993fb862f71b41b22e45e8c560332c53d8f621
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2531175Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826158}

third_party/blink/web_tests/webgpu/README.md

-To roll the WebGPU CTS forward to the latest top-of-tree, use
-`roll_webgpu_cts.sh`. This does not regenerate `wpt_internal/webgpu/cts.html`.
+# WebGPU CTS Testing
 
-To regenerate `wpt_internal/webgpu/cts.html` (after a roll, or after changes
-in WebGPUExpectations), run `regenerate_internal_cts_html.sh`.
+The WebGPU conformance test suite (CTS) is developed at
+<https://github.com/gpuweb/cts>. It is written in TypeScript and compiled to
+JavaScript to run as part of WPT. It currently has two branches:
 
-See the comments in those scripts for more info.
+- `main`: Tests which do not use GLSL shaders.
+- `glsl-dependent`: With additional tests that use GLSL shaders compiled to SPIR-V.
+
+The `roll_webgpu_cts.sh` script in this directory rolls Chromium's
+`third_party/webgpu-cts/src/` to the latest `glsl-dependent` revision, builds
+it, and saves the built files into `.../wpt_internal/webgpu/`.
+Once this is done, `.../wpt_internal/webgpu/cts.html` must also be regenerated.
+This is done with the `regenerate_internal_cts_html.sh` script.
+This must be done after a roll and after changes to `WebGPUExpectations`.
+
+(Note: there is no copy of the WebGPU CTS in WPT. This is because browsers are
+at different stages of implementation, and it is more useful to pin a particular
+revision of the CTS rather than use the latest version.)
+
+These scripts should work on Linux and macOS.
+See the comments in those scripts for more details, and below for step-by-step
+instructions on performing a roll.
+
+## How to roll the WebGPU CTS into Chromium
+
+1. Merge the cts `main` branch into `glsl-dependent` if it hasn't been
+    done recently (or ask kainino@ to do this).
+    1. Wait for
+        [Chromium's mirror](https://chromium.googlesource.com/external/github.com/gpuweb/cts/+log/refs/heads/glsl-dependent)
+        to pick up the changes (_usually_ &lt;10 minutes).
+1. Run `third_party/blink/web_tests/webgpu/roll_webgpu_cts.sh`.
+1. Repeat until regeneration succeeds:
+    1. Run `third_party/blink/web_tests/webgpu/regenerate_internal_cts_html.sh`.
+    1. In `third_party/blink/web_tests/WebGPUExpectations`,
+        delete any expectations that caused regeneration errors
+        (or rename them if you can figure out what the name change was).
+1. Commit changes, upload patch (ignore line-length warnings in generated files).
+1. Run these tryjobs: `dawn-.*-deps-rel`.
+1. Make sure there isn't anything terribly wrong
+    (e.g. a harness bug that causes all tests to fail, or not run at all).
+1. Remove stale expectations:
+    1. Look at the output of `webgpu_blink_web_tests` (and related)
+        on those tryjobs.
+    1. Remove any expectations for "passed unexpectedly" test variants
+        in `WebGPUExpectations`.
+1. Repeat until CQ passes:
+    1. Look at output of `webgpu_blink_web_tests` on all bots
+    1. Look at the output of `webgpu_blink_web_tests` (and related)
+        on any failing bots.
+    1. Add `WebGPUExpectations` lines for any test variants that
+        "failed unexpectedly" on any tryjob.
+        If they failed on all tryjobs, add them to the "Untriaged" section.
+        If they failed on a specific tryjob, add them to a platform-specific section.
+        1. Optionally, make some expectations more precise than
+            a whole file; **re-run `regenerate_internal_cts_html.sh`
+            to automatically subdivide tests to fulfill the expectations**.
+        1. If a test variant times out simply because it's very long,
+            add a Pass expectation for one of its immediate children
+            under "Test file splits."
+            (TODO: Try to figure out if we can "ping" the harness to prevent
+            timeouts due to there being many CTS test cases in one WPT variant.)
+    1. Run `dawn-.*-deps-rel`.
+1. Get a review and land the CL!
+
+## Testing Locally
+
+This is not necessary for the roll process, but if you want to run a test
+locally with `--enable-unsafe-webgpu`, you can easily do so here:
+
+*   <https://gpuweb-cts-glsl.github.io/standalone/> (`glsl-dependent` branch)
+*   <https://gpuweb.github.io/cts/standalone/> (`main` branch)