Set is_test_apk=1 for content_browsertests_apk.

Set is_test_apk=1 for content_browsertests_apk.

Without this set, you won't get the android_tools target
built when building only content_browsertests_apk, which is needed
in order to run the test on a device.

In WebRTC, I'm adding buildbots using that target for building and
running content_browsertests in https://codereview.chromium.org/26738003/

I would like to avoid having to specify a separate target in
android_all.gyp just to get this build running, and I think all targets
should build their necessary dependencies.

TEST=local building of content_browsertests_apk + verifying the
host_forwarder was built.
BUG=305749

Review URL: https://codereview.chromium.org/27622005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229632 0039d316-1c4b-4281-b951-d872f2087c98