DevTools: fix flakiness DevToolsBeforeUnloadTest.TestDevToolsOnDevTools

Fixing race condition in test.

BUG=321359
TBR=pfeldman

Review URL: https://codereview.chromium.org/77053002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236424 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236424