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Commit 7fb8b995 attempted to enable an unused thread check in
extensions::HidDeviceManager but this was not correct because while this
class is normally used from the FILE thread this is not where it is
created (issue 417939). This change adopts the pattern used in
extensions::ApiResourceManager where the BrowserContextKeyedAPI
implements its own thread assertions.

BUG=

Review URL: https://codereview.chromium.org/604843003

Cr-Commit-Position: refs/heads/master@{#297013}