Include content_shell.pak with the webkit_unit_tests_apk as internal asset.

With the CL, https://codereview.chromium.org/456413002, we are migrating from embedding inline resources
inside blink towards using chromium's grd resource system. As part of this effort, most of the tests
would require to use the 'pak' files packaged as android application assets.

These assets need to be extracted to the application data directory during the startup of the test
application. The native libraries would be able to work with these 'pak' files only after the extraction
from the asset manager. Adding the required 'content_shell.pak' to the target webkit_unit_tests_apk.

BUG=312586

Review URL: https://codereview.chromium.org/549863002

git-svn-id: svn://svn.chromium.org/blink/trunk@181556 bbb929c8-8fbe-4397-9dbb-9b2b20218538