Make backend destructor public

This allows Backend pointers to point to derived classes.

BUG=648701

Review-Url: https://codereview.chromium.org/2365283003
Cr-Original-Commit-Position: refs/heads/master@{#421221}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: fb4bbebb606d40493cdc89baecb7139c6b7c8be4
1 file changed