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The list of optional arguments is growing. Each new added one, required changes
in tens of places.

To resolve this problem, this CL creates a MountOptions struct. As a result
adding a new optional argument to mounting will not require updating all
callers.

TEST=unit_tests: *FileSystemProvider*
BUG=248427

Review URL: https://codereview.chromium.org/624903002

Cr-Commit-Position: refs/heads/master@{#299434}