canvas: check for strings before other types

Strings seem to be more common, so check for a string first.

Bug: none

Change-Id: I4671695e13e9acaa8f272944070a4fce3a306c18
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4234904


Reviewed-by: Justin Novosad <junov@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1103459}