[Paint Preview] Split PaintPreviewCompositor memory metrics from the general...

[Paint Preview] Split PaintPreviewCompositor memory metrics from the general Utility process metrics

Change-Id: I6067aad3959c054181389c1e1b82e464eacd9a4b
Bug: 1135217
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2441263


Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Reviewed-by: Calder Kitagawa <ckitagawa@chromium.org>
Commit-Queue: Yashar Dabiran <yashard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815285}