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Previously CdmHostFiles::InitVerification() was called within the
sandbox which doesn't work well on Windows. Move the call up so it's
called out of the sandbox on Windows. On other platforms this is still
called within the sandbox.

Also rename VerifyFiles() to InitVerification() to better reflect
what the call does.

See http://shortn/_lAkREmcQYG and http://shortn/_o4p8OPylLu on how
init verification is implemented in the CDM.

BUG=709614
TEST=Manually tested on Windows; Existing browser_tests still works.

Review-Url: https://codereview.chromium.org/2849163003
Cr-Commit-Position: refs/heads/master@{#468564}