Mojo: MultiprocessTestHelper: Allow {ASSERT,EXPECT}_...() to be used in child.

Add a new macro, MOJO_MULTIPROCESS_TEST_CHILD_TEST() to define the test
body in the child process, where ASSERT_...() and EXPECT_...() can be
used as expected. Add a new method, WaitForChildTestShutdown(), to
MultiprocessTestHelper, which returns true if everything passed in the
child. (Keep the old macro and old WaitForChildShutdown() around, since
sometimes we want to use the child's exit code to report other
information.)

R=yzshen@chromium.org

Review URL: https://codereview.chromium.org/194593007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256373 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256373