Add build flag to disable hotwording.

Add build flag to disable hotwording.

Hotwording downloads a shared module from the web store containing a NaCl module. There is a desire to build and distribute Chromium without this happening. This change adds an "enable_hotwording" build flag that is enabled by default, but can be disabled at compile time.

BUG=491435
Review URL: https://codereview.chromium.org/1160243004

Cr-Commit-Position: refs/heads/master@{#333548}
(cherry picked from commit f269d3b548203e217e8c0080c2e22e7ae3efb51e)

Resolved merge conflict: Remove the body of
IsHotwordAllowedDisabledFieldTrial and IsHotwordAllowedInvalidFieldTrial
when ENABLE_HOTWORDING is false. These tests were deleted before the
upstream patch landed.

(Merge approval on http://crbug.com/500922#c69)
TBR=thestig@chromium.org,scottmg@chromium.org

[Matt Giuca, 2015-06-29 01:25:52]

mgiuca@chromium.org changed reviewers:
+ scottmg@chromium.org, thestig@chromium.org

[Matt Giuca, 2015-06-29 01:26:48]

mgiuca@chromium.org changed reviewers:
+ amistry@chromium.org

[Matt Giuca, 2015-06-29 01:57:20]

Committed patchset #2 (id:20001) to pending queue manually as
837e54aaea52eec44b0108a9939f7c88ab26268a (presubmit successful).