DevTools: Fix missing samples in CPU profile.

When v8 is unable to unwind a JS stack trace it emits a sample
with an empty stack. That causes DT frontend to show gaps in stack frames.

The patch workarounds these by replacing a missing sample with a neighbor one.

It basically reverts https://chromium.googlesource.com/chromium/src/+/357c6a05e490f5e187dbad88aea96722a72c7446

Change-Id: I16f21a06bb33db160213319d38243c77ea497b09
Reviewed-on: https://chromium-review.googlesource.com/967466
Commit-Queue: Alexei Filippov <alph@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544252}

third_party/WebKit/LayoutTests/http/tests/devtools/profiler/cpu-profiler-fix-missing-samples-expected.txt

+Tests missing samples are replaced with neighbor stacks.
+Profile tree:
+(root) id:1 total:16 self:0 depth:-1
+  (garbage collector) id:2 total:3.2 self:3.2 depth:0
+  (program) id:3 total:3.2 self:3.2 depth:0
+  bar id:4 total:6.4 self:3.2 depth:0
+    foo id:6 total:3.2 self:3.2 depth:1
+  baz id:5 total:3.2 self:3.2 depth:0
+raw samples: 3  4  3  4  3  6  2  2  3  6  3  3  6  5  3  6
+samples:     3  4  4  4  4  6  2  2  3  6  3  3  6  5  3  6
+timestamps: 1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17
+forEachFrame iterator structure:
++ 0 (program) 1
+- 0 (program) 1 1 1
++ 0 bar 2
+  + 1 foo 6
+    + 2 (garbage collector) 7
+    - 2 (garbage collector) 7 2 2
+  - 1 foo 6 3 1
+- 0 bar 2 7 4
++ 0 (program) 9
+- 0 (program) 9 1 1
++ 0 bar 10
+  + 1 foo 10
+  - 1 foo 10 1 1
+- 0 bar 10 1 0
++ 0 (program) 11
+- 0 (program) 11 2 2
++ 0 bar 13
+  + 1 foo 13
+  - 1 foo 13 1 1
+- 0 bar 13 1 0
++ 0 baz 14
+- 0 baz 14 1 1
++ 0 (program) 15
+- 0 (program) 15 1 1
++ 0 bar 16
+  + 1 foo 16
+  - 1 foo 16 1 1
+- 0 bar 16 1 0
+

third_party/WebKit/LayoutTests/http/tests/devtools/profiler/cpu-profiler-fix-missing-samples.js

+
+ // Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+(async function() {
+  TestRunner.addResult(`Tests missing samples are replaced with neighbor stacks.`);
+  await TestRunner.loadModule('cpu_profiler_test_runner');
+  const profile = {
+    startTime: 1000,
+    endTime: 4000,
+    nodes: [
+      {id: 1, hitCount: 0, callFrame: {functionName: '(root)'}, children: [2,3,4,5]},
+      {id: 2, hitCount: 1000, callFrame: {functionName: '(garbage collector)'}},
+      {id: 3, hitCount: 1000, callFrame: {functionName: '(program)'}},
+      {id: 4, hitCount: 1000, callFrame: {functionName: 'bar'}, children: [6]},
+      {id: 5, hitCount: 1000, callFrame: {functionName: 'baz'}},
+      {id: 6, hitCount: 1000, callFrame: {functionName: 'foo'}}
+    ],
+    samples: [3, 4, 3, 4, 3, 6, 2, 2, 3, 6, 3, 3, 6, 5, 3, 6]
+  };
+  profile.timeDeltas = profile.samples.map(_ => 1000);
+  profile.endTime = profile.startTime + profile.timeDeltas.length * 1000;
+  const rawSamples = profile.samples.slice();
+  const model = new SDK.CPUProfileDataModel(profile);
+  TestRunner.addResult('Profile tree:');
+  printTree('', model.profileHead);
+  function printTree(padding, node) {
+    TestRunner.addResult(
+        `${padding}${node.functionName} id:${node.id} total:${node.total} self:${node.self} depth:${node.depth}`);
+    node.children.sort((a, b) => a.id - b.id).forEach(printTree.bind(null, padding + '  '));
+  }
+  TestRunner.addResult('raw samples: ' + rawSamples.join('  '));
+  TestRunner.addResult('samples:     ' + model.samples.join('  '));
+  TestRunner.addResult('timestamps: ' + model.timestamps.join('  '));
+  TestRunner.addResult('forEachFrame iterator structure:');
+  model.forEachFrame(
+    (depth, node, ts) =>
+      TestRunner.addResult('  '.repeat(depth) + `+ ${depth} ${node.callFrame.functionName} ${ts}`),
+    (depth, node, ts, total, self) =>
+      TestRunner.addResult('  '.repeat(depth) + `- ${depth} ${node.callFrame.functionName} ${ts} ${total} ${self}`),
+  );
+  TestRunner.completeTest();
+})();

third_party/WebKit/Source/devtools/front_end/sdk/CPUProfileDataModel.js

@@ -58,6 +58,7 @@ SDK.CPUProfileDataModel = class extends SDK.ProfileTreeModel {
       this._buildIdToNodeMap();
       this._sortSamples();
       this._normalizeTimestamps();
+      this._fixMissingSamples();
     }
   }
 
@@ -246,6 +247,53 @@ SDK.CPUProfileDataModel = class extends SDK.ProfileTreeModel {
     }
   }
 
+  _fixMissingSamples() {
+    // Sometimes sampler is not able to parse the JS stack and returns
+    // a (program) sample instead. The issue leads to call frames belong
+    // to the same function invocation being split apart.
+    // Here's a workaround for that. When there's a single (program) sample
+    // between two call stacks sharing the same bottom node, it is replaced
+    // with the preceeding sample.
+    const samples = this.samples;
+    const samplesCount = samples.length;
+    if (!this.programNode || samplesCount < 3)
+      return;
+    const idToNode = this._idToNode;
+    const programNodeId = this.programNode.id;
+    const gcNodeId = this.gcNode ? this.gcNode.id : -1;
+    const idleNodeId = this.idleNode ? this.idleNode.id : -1;
+    let prevNodeId = samples[0];
+    let nodeId = samples[1];
+    let count = 0;
+    for (let sampleIndex = 1; sampleIndex < samplesCount - 1; sampleIndex++) {
+      const nextNodeId = samples[sampleIndex + 1];
+      if (nodeId === programNodeId && !isSystemNode(prevNodeId) && !isSystemNode(nextNodeId) &&
+          bottomNode(idToNode.get(prevNodeId)) === bottomNode(idToNode.get(nextNodeId))) {
+        ++count;
+        samples[sampleIndex] = prevNodeId;
+      }
+      prevNodeId = nodeId;
+      nodeId = nextNodeId;
+    }
+    Common.console.warn(ls`DevTools: CPU profile parser is fixing ${count} missing samples.`);
+    /**
+     * @param {!SDK.ProfileNode} node
+     * @return {!SDK.ProfileNode}
+     */
+    function bottomNode(node) {
+      while (node.parent && node.parent.parent)
+        node = node.parent;
+      return node;
+    }
+    /**
+     * @param {number} nodeId
+     * @return {boolean}
+     */
+    function isSystemNode(nodeId) {
+      return nodeId === programNodeId || nodeId === gcNodeId || nodeId === idleNodeId;
+    }
+  }
+
   /**
    * @param {function(number, !SDK.CPUProfileNode, number)} openFrameCallback
    * @param {function(number, !SDK.CPUProfileNode, number, number, number)} closeFrameCallback