[DevTools] Network panel: add "Scheme" column.

Plus minor refactoirngs.

BUG=277611

Review URL: https://chromiumcodereview.appspot.com/23215003

git-svn-id: svn://svn.chromium.org/blink/trunk@156642 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Reland "Add trace event for MemoryCache::prune()."

The root cause of the bug 271440 has been fixed by 
https://codereview.chromium.org/23181003/.

This relands commit 432f2a91e7beedf020c36dd5dd67392f1800dd47
(https://chromiumcodereview.appspot.com/23031003)

BTW reverts commit 843811659cd2fb9b4349604c028339f5439d8a8d
(Assert webkit platform in MemoryCache::prune().
 https://chromiumcodereview.appspot.com/23102002)
as it's no longer necessary.

BUG=271440,272334

Review URL: https://chromiumcodereview.appspot.com/23394002

git-svn-id: svn://svn.chromium.org/blink/trunk@156643 bbb929c8-8fbe-4397-9dbb-9b2b20218538