Fix crash when a frame is detached while a PepperPluginInstance is being constructed.

BUG=638400

Review-Url: https://codereview.chromium.org/2322503003
Cr-Commit-Position: refs/heads/master@{#417009}
(cherry picked from commit ba4daaff922e8c3f84cb4f7553a5c30292c7e748)

TBR=creis@chromium.org
NOTRY=true
NOPRESUBMIT=true

Review URL: https://codereview.chromium.org/2369453004 .

Cr-Commit-Position: refs/branch-heads/2785@{#922}
Cr-Branched-From: 68623971be0cfc492a2cb0427d7f478e7b214c24-refs/heads/master@{#403382}
1 file changed