Fix canvas type YAML key for 2d.gradient.colormix WPT test

https://crrev.com/c/5377005 renamed the canvasType YAML key to canvas_types in the WPT canvas test generator. Then, https://crrev.com/c/5371273 was submitted using the old config format. This means that the canvasType config of the 2d.gradient.colormix test is now ignored, causing offscreen and worker test files to be unexpectedly generated.

Bug: 327837697 Change-Id: I72244f282418fc71fdf5e339295f0f952f12bb41 Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5425461 Auto-Submit: Jean-Philippe Gravel jpgravel@chromium.org Reviewed-by: Perry perryuwang@tencent.com Commit-Queue: Perry perryuwang@tencent.com Cr-Commit-Position: refs/heads/main@{#1283069}