DevTools: don't fail on opening devtools when there is no screencast available.
DevTools: don't fail on opening devtools when there is no screencast available.

R=apavlov@chromium.org

Review URL: https://codereview.chromium.org/26246002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@159027 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs