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

This reverts commit ea7499df.

Reason for revert: Speculative, but this CL seems correlated with a bunch of timeouts on Linux:

https://build.chromium.org/p/client.v8/builders/V8%20Linux%20-%20nosnap/builds/19540

Original change's description:
> [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/844979
> Reviewed-by: Yang Guo <yangguo@chromium.org>
> Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#50327}

TBR=yangguo@chromium.org,kozyatinskiy@chromium.org

Change-Id: I543201698c96c9762c481c1f6012cc13cb712842
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:797573, chromium:792838
Reviewed-on: https://chromium-review.googlesource.com/846205Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50331}

src/debug/debug-evaluate.cc

@@ -58,6 +58,13 @@ 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