Stop image CSS sheet overriding any other sheets.
Stop image CSS sheet overriding any other sheets.

This doesn't cause us issues at the moment, but it is possible in the
future that we may adopt more stylesheets onto the document. This CL
updates so we always append the sheet rather than just overriding any
existing stylesheets.

Bug: none
Change-Id: I7f7abca19f1e4e5e40234a263aaddbb79da9286b
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/3090586
Auto-Submit: Jack Franklin <993fcadce4d04090da2fefd557a0995e7966c8d5@chromium.org>
Commit-Queue: Tim van der Lippe <dba8716ee7f8d16236046f74d2167cb94410f6ed@chromium.org>
Reviewed-by: Tim van der Lippe <dba8716ee7f8d16236046f74d2167cb94410f6ed@chromium.org>

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