This fixes a problem when the RenderFrame is destroyed within the context of a getusermedia callback.

      BUG=472617, 474370
TBR=tommi@chromium.org

Review URL: https://codereview.chromium.org/1075833002

Cr-Commit-Position: refs/heads/master@{#324434}
(cherry picked from commit fa7c6fd4aa278dab68c5987b45353e064ce8e0ad)

Review URL: https://codereview.chromium.org/1088923004

Cr-Commit-Position: refs/branch-heads/2357@{#82}
Cr-Branched-From: 59d4494849b405682265ed5d3f5164573b9a939b-refs/heads/master@{#323860}