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Add a TestExtensionDir::CopyFileTo() method that copies a given
filepath into the temporary directory held by the TestExtensionDir. This
prevents callers from having to either do the copy more manually
(reading the content and then writing it) or poking into the
TestExtensionDir's inner temp directory.

Bug: None
Change-Id: I3ce727548ff9a4184c33d57bbacaee7e132baa43
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2265292Reviewed-by: David Bertoni <dbertoni@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#782260}