Mac: Replace deprecated use of ::FSFindFolder() in plugin_list_mac.mm
Mac: Replace deprecated use of ::FSFindFolder() in plugin_list_mac.mm

Alternatives are
- -[NSFileManager URLForDirectory:inDomain:appropriateForURL:create:error:]
- -[NSFileManager URLsForDirectory:inDomains:]
- ::NSSearchPathForDirectoriesInDomains(..)

Documentation for the latter suggests "You should consider using the
NSFileManager methods"

Unfortunately, these all take a NSSearchPathDirectory constant, and
there is no constant for `[~]/Library/Internet Plug-Ins`. Safari has
hard coded the path relative to $HOME (or /) ever since
plugin_list_mac.mm used FSFindFolder, so let's start there.

The FindFolder function returns a nonzero result code if the folder
isn't found, so retain that behaviour by checking for existence
(although the result is passed to base::FileEnumerator which doesn't
really care). The NSFileManager methods don't actually document wether
they return non-existent folders, but empirically they will, and
NSSearchPathForDirectoriesInDomains says it may.

Traced manually to ensure the same paths are produced; in the same order.

BUG=547071

Review URL: https://codereview.chromium.org/1841243002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#384076}

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