[Merge] Pepper: Access PepperMediaDeviceManager through a WeakPtr

Its lifetime is scoped to the RenderFrame, and it might go away before the
hosts that refer to it.

BUG=423030

Committed: https://crrev.com/3a2cf7d1376ae33054b878232fb38b8fbed29e31
Cr-Commit-Position: refs/heads/master@{#299897}

TBR=teravest@chromium.org

Review URL: https://codereview.chromium.org/653243003

Cr-Commit-Position: refs/branch-heads/2171@{#245}
Cr-Branched-From: 267aeeb8d85c8503a7fd12bd14654b8ea78d3974-refs/heads/master@{#297060}
(cherry picked from commit 14ea095e585e23aa073c1434bd75d6a3e055836b)

Review URL: https://codereview.chromium.org/702783002

Cr-Commit-Position: refs/branch-heads/2125@{#599}
Cr-Branched-From: b68026d94bda36dd106a3d91a098719f952a9477-refs/heads/master@{#290040}
7 files changed