DevTools: Fix autocomplete for this expressions on pause

`ThisExpression` is not considered an `Identifier`, but it should
similarly have no side effects.

Change-Id: I894869716541dd9692d96938db76b2b3a1d3b1a1
Reviewed-on: https://chromium-review.googlesource.com/1028187Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Joel Einbinder <einbinder@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553839}

third_party/WebKit/LayoutTests/http/tests/devtools/sources/debugger/debugger-completions-on-call-frame-expected.txt

@@ -46,5 +46,8 @@ PASS
 
 Running: step15_ShouldNotCrash
 PASS
+
+Running: step16
+PASS
 Script execution resumed.
 

third_party/WebKit/LayoutTests/http/tests/devtools/sources/debugger/debugger-completions-on-call-frame.js

@@ -93,6 +93,11 @@
     function step15_ShouldNotCrash(next) {
       ObjectUI.javaScriptAutocomplete._completionsForExpression('arr3.', '')
           .then(checkAgainstGolden.bind(this, ['length'], ['1', '2', '3'], next));
+    },
+
+    function step16(next) {
+      ObjectUI.javaScriptAutocomplete._completionsForExpression('this.', 'win')
+          .then(checkAgainstGolden.bind(this, ['window'], ['arr3'], next));
     }
   ]);
 

third_party/blink/renderer/devtools/front_end/formatter_worker/FormatterWorker.js

@@ -373,7 +373,7 @@ FormatterWorker.findLastExpression = function(content) {
     baseExpression = `(${baseExpression})`;
   const sideEffectFreeTypes = new Set([
     'MemberExpression', 'Identifier', 'BinaryExpression', 'Literal', 'TemplateLiteral', 'TemplateElement',
-    'ObjectExpression', 'ArrayExpression', 'Property'
+    'ObjectExpression', 'ArrayExpression', 'Property', 'ThisExpression'
   ]);
   let possibleSideEffects = false;
   const sideEffectwalker = new FormatterWorker.ESTreeWalker(node => {