Add a context_ null check needed in case of DeleteAndStartOver.

      BUG=448082

Review URL: https://codereview.chromium.org/798883005

Cr-Commit-Position: refs/heads/master@{#311308}
(cherry picked from commit 55f7c1cace6d82a071bccf8a465944c5e63a3e2a)

Review URL: https://codereview.chromium.org/949603002

Cr-Commit-Position: refs/branch-heads/2272@{#350}
Cr-Branched-From: 827a380cfdb31aa54c8d56e63ce2c3fd8c3ba4d4-refs/heads/master@{#310958}