Verify that gin_shell and hello_world.js exist.
Verify that gin_shell and hello_world.js exist.

This issue: https://code.google.com/p/chromium/issues/detail?id=398549,
was difficult to debug because gin/shell/gin_shell_unittest.cc didn't verify
that its input files existed before forging ahead.

The unit test now verifies that the gin_shell and hello_world.js paths
exist before launching the former and loading the latter.


BUG=398549

Review URL: https://codereview.chromium.org/471623002

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@289599 0039d316-1c4b-4281-b951-d872f2087c98

littlstar/chromium.src,bright-sparks/chromium-spacewalk,ondra-novak/chromium.src,ondra-novak/chromium.src,ondra-novak/chromium.src,ondra-novak/chromium.src,bright-sparks/chromium-spacewalk,littlstar/chromium.src,littlstar/chromium.src,littlstar/chromium.src,ondra-novak/chromium.src,ondra-novak/chromium.src,bright-sparks/chromium-spacewalk,bright-sparks/chromium-spacewalk,littlstar/chromium.src,bright-sparks/chromium-spacewalk,ondra-novak/chromium.src,bright-sparks/chromium-spacewalk,littlstar/chromium.src,ondra-novak/chromium.src,ondra-novak/chromium.src,littlstar/chromium.src,bright-sparks/chromium-spacewalk,bright-sparks/chromium-spacewalk,bright-sparks/chromium-spacewalk,littlstar/chromium.src