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Content size notification was only used for the Mac overlay navigation code, which no longer needs it as of Chromium r274906.

Remove it.

BUG=321437
TEST=everything still compiles and works.

Review URL: https://codereview.chromium.org/322793004

git-svn-id: svn://svn.chromium.org/blink/trunk@175861 bbb929c8-8fbe-4397-9dbb-9b2b20218538