As mentioned in the base::Value refactor proposal, base::Value::Type should be a
C++11 style scoped enumeration. This change addresses this issue by adding the
required keyword, making the necessary substitutions and adding casts where
needed.

TBR=stevenjb@chromium.org
BUG=646113
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2539363004
Cr-Commit-Position: refs/heads/master@{#436961}