Contextual search on Android spins up a WebContents which talks to
Google search. If Google search chooses to request a permission inside
this WebContents, the modal prompt crashes due to a lack of a containing
tab. This is not an issue with the standard infobar permission prompts
as they do not have a dependency on a tab for display.

This CL changes a DCHECK for a TabAndroid to an explicit conditional,
which acts as though the permission request was dismissed if there is no
active Tab for the requesting WebContents.

BUG=668640

Review-Url: https://codereview.chromium.org/2538253002
Cr-Commit-Position: refs/heads/master@{#435556}