[extension bindings] Remove leak of internal class

BUG=660678

Review-Url: https://codereview.chromium.org/2474623002
Cr-Commit-Position: refs/heads/master@{#429482}
(cherry picked from commit 0d9edd7b2524090fb0a0461e7bd9e5caa1e8a36d)

Review URL: https://codereview.chromium.org/2476303002 .

Cr-Commit-Position: refs/branch-heads/2883@{#474}
Cr-Branched-From: 614d31daee2f61b0180df403a8ad43f20b9f6dd7-refs/heads/master@{#423768}
1 file changed