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Old name was presumably used to match was_within_same_page IPC, but
SameDocument name better reflects the the type of the navigation
(navigation did not change the document object).

This change will make naming more consistent with the rest of Chromium
code.

BUG=695189
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2841833002
Cr-Commit-Position: refs/heads/master@{#467720}