Add PSS.getActiveDataTypes().

Add PSS.getActiveDataTypes().

Currently PSS.getPreferredDataTypes actually returns
PSS::getActiveDataTypes(), which causes some problems. This CL is 1/3.
The second will change the appropriate downstream calls to
getActiveDataTypes() and the third will fix getPreferredDataTypes().

BUG=466223
TBR=maxbogue@chromium.org

Review URL: https://codereview.chromium.org/1017683005

Cr-Commit-Position: refs/heads/master@{#321217}
(cherry picked from commit 86e260e3374e107306d8759e78f886a5f5c0de6e)

Committed: https://chromium.googlesource.com/chromium/src/+/da2ce9b1ab173b812f31b513c0c69e57cbe41740

[Nicolas Zea, 2015-04-01 20:21:37]

zea@chromium.org changed reviewers:
+ maxbogue@chromium.org

[Nicolas Zea, 2015-04-01 20:24:40]

Committed patchset #1 (id:1) manually as
da2ce9b1ab173b812f31b513c0c69e57cbe41740 (presubmit successful).