Copy blink_testing target into standalone gyp file to evade cycles

content/content_shell.gypi, which is included into content/content.gyp,
depends on public/all.gyp:blink_testing. The gyp generator on OS X forbids
any cycles between referenced gyp files, so this means that nothing
referenced by all.gyp can reference anything included into content.gyp.
In https://codereview.chromium.org/18711009/, I want to make webkit_unit_tests
depend on a test library in content/content_tests.gypi but that's also
included into content.gyp. There's no cycle at the target level, only at the
gyp file level, so this splits out the test target into
public/blink_test_runner.gyp:blink_test_runner. I changed the name to avoid
having duplicate target names in the transition period and to make it clearer how
this library is different from the libraries used for unit tests.

R=abarth

Review URL: https://chromiumcodereview.appspot.com/19485002

git-svn-id: svn://svn.chromium.org/blink/trunk@154366 bbb929c8-8fbe-4397-9dbb-9b2b20218538