Perform quiet mutations on StyleSheetContents copy, if needed

InspectorGhostRules quietly modifies stylesheets without invalidation,
because we're guaranteed to end up back with the original state.
It did not, however, account for the fact that multiple
StyleSheetContents objects can be shared between CSSStyleSheets.
Even "quiet" mutations must respect copy-on-write in that case,
which is what this CL does: when we're about to quietly insert
a rule, we first ensure that the StyleSheetContents isn't shared.
This may cause the StyleSheetContents to copied, with the original
contents held onto and ultimately restored by QuietMutationScope.

Possible future improvements:

 - Caching the copied StyleSheetContents somewhere in InspectorCSSAgent.
   Might not help that much within a single call to
   getMatchedStylesForNode, but could help across multiple such calls.
 - Using QuietMutationScope as a "pass key" to the existing
   Quiet* API, to ensure that quiet mutations indeed take place during
   that scope.

Fixed: 389011795
Change-Id: I37acfcf89c39e43575befb26cb395246e041d34c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6361495
Commit-Queue: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1434014}