Debugger: use FrameInspector in ScopeIterator to find context.

In optimized code, it's not guaranteed that the current context
is stored in its frame slot.

R=bmeurer@chromium.org
BUG=v8:4309
LOG=N

Committed: https://crrev.com/3a0ee39cbde6a9778cfc4e2a6a0a8ff68933ff38
Cr-Commit-Position: refs/heads/master@{#29697}

Review URL: https://codereview.chromium.org/1239033002

Cr-Commit-Position: refs/heads/master@{#29744}