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This CL introduces a FileManager class for managing the top level
paint_preview/ directory and the immediate per-url subdirectories.
All creation/deletion of these directories should occur through this
class. However, files within the per-url subdirectories can be modified
by any component of paint_preview (primarily the capture and
compositing code).

This class will be an integral component of a higher level manager
class. The manager will call into this class when it needs to perform
a capture or do deletion.

Bug: 1010042
Change-Id: I7b45a8f730837fcc1d0262ab0dcf3c89698974f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1833289
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Reviewed-by: Ian Vollick <vollick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#701702}