Don't call WebContents::DownloadImage() callback if the WebContents were deleted

BUG=583718

Review URL: https://codereview.chromium.org/1685343004

Cr-Commit-Position: refs/heads/master@{#375700}
(cherry picked from commit 75ca8ffd7bd7c58ace1144df05e1307d8d707662)

Review URL: https://codereview.chromium.org/1730363003 .

Cr-Commit-Position: refs/branch-heads/2623@{#504}
Cr-Branched-From: 92d77538a86529ca35f9220bd3cd512cbea1f086-refs/heads/master@{#369907}
2 files changed