Fix crash on null WebFrame::dataSource
Fix crash on null WebFrame::dataSource

During frame detach the data source can be null. This needs to be
checked for in case a DidFlushPaint message is handled.

TBR=jam@chromium.org
BUG=466368

Review URL: https://codereview.chromium.org/1115793003

Cr-Commit-Position: refs/heads/master@{#327835}
(cherry picked from commit 9548660d3efe07c8731a28a1df4140964f753bb9)

Review URL: https://codereview.chromium.org/1137433005

Cr-Commit-Position: fe94940391158563b8e4cf7888f93321f554af93@{#375}
Cr-Branched-From: 9fdd9ec11ee753dedeffe15e7e822b13445dc287@{#323860}

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