Make CustomThemeSupplier::GetImageNamed() const

This CL makes CustomThemeSupplier::GetImageNamed() const by marking
BrowserThemePack's images_ cache as mutable.
This is to stop relying on an implicit const cast via
scoped_refptr<T>::operator->() const returning a non-const T*.
This is in preparation for:
https://chromium-review.googlesource.com/c/chromium/src/+/1994026

Bug: 1020050
Change-Id: Ifb604065f926bc4d922112225018e0ff31d1e8a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2006283
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733215}
5 files changed