Merge WebUSB notification flag into experimental web platform features.

The WebUSB notification flag tells Chrome to scan for WebUSB-enabled
devices when they are connected to the system. This patch will expose
this code to a larger population to make sure that such scanning does
not introduce new crashes and reduce the number of flags developers must
enable to try out WebUSB.

BUG=492204

Review URL: https://codereview.chromium.org/1426583008

Cr-Commit-Position: refs/heads/master@{#357639}
5 files changed