The "easy off-store install" switch is used to determine whether or not
chrome will automatically install an extension that isn't hosted in the
webstore if the user downloads it. However, this has been disabled on
all platforms, and has no associated commandline switch or flag, so it
is permanently disabled. The only usages are in tests.

Remove the feature switch and add a lighter-weight test function to
accommodate.

Also re-enable an effectively-disabled extension browsertest (which only
ran when the easy off-store install switch was enabled - and that was
never true).

BUG=687623
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2668863002
Cr-Commit-Position: refs/heads/master@{#448311}