Add database for Dom Distiller articles

For the dom distiller, there is a local list of article metadata. 
This adds a skeleton proto for the dom distiller articles, and provides the
implementation of a database for persisting the entries. Underlying
database is provided by leveldb.

Also adds unit test for both the asynchronous communication with the
underlying database and tests for the underlying usage of leveldb.

BUG=295840,288015
TBR=joi@chromium.org

Review URL: https://chromiumcodereview.appspot.com/22799030

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224648 0039d316-1c4b-4281-b951-d872f2087c98
Remove locks from leveldb's stdio on linux/CrOS.

After being in stable for a few weeks it has not found any unsafe thread
usage.

BUG=230154

Review URL: https://chromiumcodereview.appspot.com/23440039

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224649 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224649
[Android] EME permission on/off pref and infobar UI.

For protected media identifier access permission,
add global on/off preference setting and infobar permission gate UI.

BUG=281576

Review URL: https://chromiumcodereview.appspot.com/23531021

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224650 0039d316-1c4b-4281-b951-d872f2087c98
Enable chrome-for-tv override default key systems

BUG=

Review URL: https://chromiumcodereview.appspot.com/23607004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224651 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224650
SVN changes up to revision 224651