[GTK] Invalidate frame image cache when dark preference changes

WindowFrameProviderGtk was only checking for the theme name and not
the dark mode preference.  This CL switches to using signals to
invalidate the cache and also removes scale factor rounding to the
nearest 1/100th since the scale is already rounded to the nearest
1/64th elsewhere.

R=sky

Change-Id: I840b168f6dd47eec084ecbaffdf19a87fa4f6049
Bug: None
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5191808
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1246605}