base: Clarify that MAIN_THREAD_MOCK_TIME affects base::Time::Now()

When using ScopedTaskEnvironment::NowSource::MAIN_THREAD_MOCK_TIME, both
base::Time::Now() and base::TimeTicks::Now() are affected. However, this
isn't clear in the comment and there's no unit test covering it.

This CL updates the comment and added tests to cover both
base::Time::Now() and base::TimeTicks::Now().

Bug: 917527
Test: New tests added.
Change-Id: Ida9b5f334e12e0bceb9e952f97f0ee9ef01ffc45
Reviewed-on: https://chromium-review.googlesource.com/c/1450402
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628456}

base/test/scoped_task_environment.h

@@ -100,11 +100,12 @@ class ScopedTaskEnvironment {
   };
 
   enum class NowSource {
-    // base::TimeTicks::Now is real time.
+    // base::Time::Now() and base::TimeTicks::Now() are real time.
     REAL_TIME,
 
-    // base::TimeTicks::Now is driven from the main thread's MOCK_TIME. This
-    // may alter the order of delayed and non-delayed tasks on other threads.
+    // base::Time::Now() and base::TimeTicks::Now() are driven from the main
+    // thread's MOCK_TIME. This may alter the order of delayed and non-delayed
+    // tasks on other threads.
     //
     // Warning some platform APIs are still real time, and don't interact with
     // MOCK_TIME as expected, e.g.:

base/test/scoped_task_environment_unittest.cc

@@ -322,6 +322,28 @@ TEST_F(ScopedTaskEnvironmentTest, FastForwardAdvanceMockClock) {
   EXPECT_EQ(start_time + kDelay, clock->Now());
 }
 
+TEST_F(ScopedTaskEnvironmentTest, FastForwardAdvanceTime) {
+  constexpr base::TimeDelta kDelay = TimeDelta::FromSeconds(42);
+  ScopedTaskEnvironment scoped_task_environment(
+      ScopedTaskEnvironment::MainThreadType::MOCK_TIME,
+      ScopedTaskEnvironment::NowSource::MAIN_THREAD_MOCK_TIME);
+
+  const Time start_time = base::Time::Now();
+  scoped_task_environment.FastForwardBy(kDelay);
+  EXPECT_EQ(start_time + kDelay, base::Time::Now());
+}
+
+TEST_F(ScopedTaskEnvironmentTest, FastForwardAdvanceTimeTicks) {
+  constexpr base::TimeDelta kDelay = TimeDelta::FromSeconds(42);
+  ScopedTaskEnvironment scoped_task_environment(
+      ScopedTaskEnvironment::MainThreadType::MOCK_TIME,
+      ScopedTaskEnvironment::NowSource::MAIN_THREAD_MOCK_TIME);
+
+  const TimeTicks start_time = base::TimeTicks::Now();
+  scoped_task_environment.FastForwardBy(kDelay);
+  EXPECT_EQ(start_time + kDelay, base::TimeTicks::Now());
+}
+
 #if defined(OS_WIN)
 // Regression test to ensure that ScopedTaskEnvironment enables the MTA in the
 // thread pool (so that the test environment matches that of the browser process