Rewrites SerializedNavigationEntry to not have any //content member variables.

This will pave the way for a future CL that completely removes all //content
dependencies from the class.

TBR=ben@google.com
BUG=371476
TEST=No visible impact.

Review URL: https://codereview.chromium.org/658073004

Cr-Commit-Position: refs/heads/master@{#300344}
10 files changed