chore: cherry-pick 1eb1e18ad41d from chromium

[nornagon]

Add CSSTokenizer-created strings to CSSVariableData's backing strings

When computing the value of a registered custom property, we create
a CSSVariableData object equivalent to the computed CSSValue by
serializing that CSSValue to a String, then tokenizing that value.

The problem is that CSSTokenizer can create new string objects
during the tokenization process (see calls to CSSTokenizer::
RegisterString), without communicating that fact to the call-site.

Therefore, this CL adds a way to access those strings so they can
be added to the backing strings of the CSSVariableData.

Also added a DCHECK to verify that we don't have any tokens with
non-backed string pointers.

Fixed: 1358907
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3892782
Reviewed-by: Steinar H Gunderson sesse@chromium.org
Commit-Queue: Anders Hartvoll Ruud andruud@chromium.org
Cr-Original-Commit-Position: refs/heads/main@{#1046868}
Change-Id: Ifb6d194508e99030a5a3ed5fbad5496b7263bdc1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3905727
Auto-Submit: Anders Hartvoll Ruud andruud@chromium.org
Cr-Commit-Position: refs/branch-heads/5249@{#518}
Cr-Branched-From: 4f7bea5de862aaa52e6bde5920755a9ef9db120b-refs/heads/main@{#1036826}

Ref electron/security#221

Notes: Security: backported fix for CVE-2022-3304.

[release-clerk]

Release Notes Persisted

Security: backported fix for CVE-2022-3304.