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}