Make HandleCrossAppNavigation() handle the case when there's no source tab.
Make HandleCrossAppNavigation() handle the case when there's no source tab.

E.g., Browser::OpenURL() will call OpenURLFromTab() (etc.) with no TabContents.

BUG=43040
TEST=Go to Prefs/Options dialog -> Under the Hood -> Privacy, click on "Learn more"; this should work and not crash.

Review URL: http://codereview.chromium.org/1917003

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@46392 0039d316-1c4b-4281-b951-d872f2087c98

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