[wasm] prototype for breakpoint support.

[wasm] prototype for breakpoint support.

This CL touches several things:
- It instantiates a WasmInterpreter on demand, with a weak reference to the
  WasmDebugInfo object, such that it gets deleted when the debug info goes
  away.
- It compiles WASM_TO_INTERPRETER code stubs on demand, and redirect wasm
  callers to these stubs.
- It adds a new code kind WASM_TO_INTERPRETER for those stubs.
- It adds a new frame type WASM_INTERPRETED, which is used when
  WASM_TO_INTERPRETER code is found to be executed in a frame.
- It adds frame inspection for this new frame type by looking into the state
  of the interpreter. This uses the same Frame::Summarize mechanism as
  optimized JS frames.
- It adds a WasmBytecodeIterator which enumerates all instruction locations
  inside a wasm function.
- It allocates a DebugInfo object per wasm function on demand, to store
  breakpoint information.
- It fixes the WasmInterpreter to only allow setting breakpoints on actual
  instruction offsets; the current implementation just overwrites an
  arbitrary byte in the function by kInternalBreakpoint.

Before being committed, it needs to be split up though, and a memory error
(showing up in GC stress tests) still has to be tracked down and eliminated.

BUG=chromium:613110