Emit UMAs for metrics from memory instrumentation service.

chrome/browser/metrics/process_memory_metrics_emitter_browsertest.cc

+// Copyright 2017 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.
+
+#include "chrome/browser/metrics/process_memory_metrics_emitter.h"
+
+#include "base/message_loop/message_loop.h"
+#include "chrome/test/base/in_process_browser_test.h"
+#include "content/public/test/test_utils.h"
+
+namespace {
+
+class ProcessMemoryMetricsEmitterFake : public ProcessMemoryMetricsEmitter {
+ public:
+  ProcessMemoryMetricsEmitterFake() {}
+
+ private:
+  ~ProcessMemoryMetricsEmitterFake() override {}
+
+  void ReceivedMemoryDump(
+      uint64_t dump_guid,

[dcheng, 2017/05/11 22:12:27]

FWIW, I still think the typical expectation for "guid" in Chrome is 128-bits of
randomness, so naming a uint64_t guid is somewhat deceiving. *shrug*

[erikchen, 2017/05/11 22:18:47]

@primiano:
thakis and I had a discussion about birthday paradox and Chrome. In general,
64-bits is not enough to guarantee a low probability of collision, over all
users, over several years. [Obviously depends on how frequently the "guid" is
generated]. 

[Nico, 2017/05/11 22:24:38]

IIRC the numbers were like "there's a 5% chance that any user has any collision
over 3 years" with 64-bit, with the failure case being "nothing bad happens". To
me that sounded like "64 bits should be plenty" and to Erik it sounded like
"64-bits are not enough" :-)

+      bool success,
+      memory_instrumentation::mojom::GlobalMemoryDumpPtr ptr) override {
+    EXPECT_TRUE(success);
+    base::MessageLoop::current()->QuitWhenIdle();
+  }
+
+  DISALLOW_COPY_AND_ASSIGN(ProcessMemoryMetricsEmitterFake);
+};
+
+}  // namespace
+
+class ProcessMemoryMetricsEmitterTest : public InProcessBrowserTest {
+ public:
+  ProcessMemoryMetricsEmitterTest() {}
+  ~ProcessMemoryMetricsEmitterTest() override {}
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(ProcessMemoryMetricsEmitterTest);
+};
+
+IN_PROC_BROWSER_TEST_F(ProcessMemoryMetricsEmitterTest, FetchAndEmitMetrics) {
+  // Intentionally let emitter leave scope to check that it correctly keeps
+  // itself alive.
+  {
+    scoped_refptr<ProcessMemoryMetricsEmitterFake> emitter(
+        new ProcessMemoryMetricsEmitterFake);
+    emitter->FetchAndEmitProcessMemoryMetrics();
+  }
+
+  content::RunMessageLoop();
+}