[debugger] do not try to find saved context before DebugEvaluate::Local

Current implementation produces crashes since sometimes saved context
is empty. It looks like we do not need to restore saved context since
we do not set debug context as current, at least all tests are passed.

R=yangguo@chromium.org

Bug: chromium:797573,chromium:792838
Change-Id: I1271640f6a18cbaaecfa1e99ed9ac28e0dbbb1da
Reviewed-on: https://chromium-review.googlesource.com/844979Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50327}

src/debug/debug-evaluate.cc

@@ -58,13 +58,6 @@ MaybeHandle<Object> DebugEvaluate::Local(Isolate* isolate,
   if (!it.is_javascript()) return isolate->factory()->undefined_value();
   JavaScriptFrame* frame = it.javascript_frame();
 
-  // Traverse the saved contexts chain to find the active context for the
-  // selected frame.
-  SaveContext* save =
-      DebugFrameHelper::FindSavedContextForFrame(isolate, frame);
-  SaveContext savex(isolate);
-  isolate->set_context(*(save->context()));
-
   // This is not a lot different than DebugEvaluate::Global, except that
   // variables accessible by the function we are evaluating from are
   // materialized and included on top of the native context. Changes to