Fix a potential crash in MidiManagerMac.

      R=toyoshim@chromium.org,agoode@chromium.org
BUG=564501

Review URL: https://codereview.chromium.org/1508563003

Cr-Commit-Position: refs/heads/master@{#363714}
(cherry picked from commit 54d256d1fc9c6a9e7438f6f51206d1f99b1ed6b4)

Review URL: https://codereview.chromium.org/1515673002 .

Cr-Commit-Position: refs/branch-heads/2526@{#519}
Cr-Branched-From: cb947c0153db0ec02a8abbcb3ca086d88bf6006f-refs/heads/master@{#352221}