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This ensures that subframe processes aren't left around if a parent page
removes an out-of-process iframes.

TBR=jam@chromium.org
BUG=471346, 474772
TEST=See bug for repro steps.
Credit to nick@ for TaskManagerOOPIFBrowserTest.LeavePageWithCrossSiteIframes

Review URL: https://codereview.chromium.org/1039403002

Cr-Commit-Position: refs/heads/master@{#324160}