Revert &quotConvert extensions::SafeBuiltins from a v8::Extension to a NativeHandler.&quot

      This reverts commit 68c06a78f74212907faafd985b0bdee67a6d4120.

It caused an iframe performance regression. I plan on re-landing this
after doing some profiling to figure out what exactly the problem is.

TBR=rdevlin.cronin@chromium.org, jochen@chromium.org
BUG=517509, 383974

Review URL: https://codereview.chromium.org/1281793003

Cr-Commit-Position: refs/heads/master@{#342858}