Refactoring: Make Resource::m_data private

This CL makes Resource::m_data private to improve code health.

After this CL, all protected member vairables in Resource will be
changed to private.

BUG=n/a
TEST=webkit_unit_tests

Review-Url: https://codereview.chromium.org/2202173002
Cr-Commit-Position: refs/heads/master@{#410013}
9 files changed