[Backport] M64: Ensure clamped time always moves forward

This patch fixes a problem where performance.now or Date.now can in rare cases
move slightly backwards due to a loss of arithmetic precision.

BUG=801341,799127,798964
TBR=skyostil@chromium.org

(cherry picked from commit 874bac6e00e5b0b5fe291cc51518e5fd1e83c69d)

Reviewed-on: https://chromium-review.googlesource.com/867062
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Sami Kyöstilä <skyostil@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#529407}
Reviewed-on: https://chromium-review.googlesource.com/882783
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/branch-heads/3282@{#588}
Cr-Branched-From: 5fdc0fab22ce7efd32532ee989b223fa12f8171e-refs/heads/master@{#520840}

Change-Id: If5bdc9933373cf320e9cf2efe8141275ef8c40c4
Reviewed-by: Michael Brüning <michael.bruning@qt.io>

2 files changed, 4 insertions, 3 deletions

diff --git a/chromium/third_party/WebKit/Source/platform/TimeClamper.cpp b/chromium/third_party/WebKit/Source/platform/TimeClamper.cpp
index 18244e769f3..2445af4fa33 100644
--- a/chromium/third_party/WebKit/Source/platform/TimeClamper.cpp
+++ b/chromium/third_party/WebKit/Source/platform/TimeClamper.cpp
@@ -18,12 +18,12 @@ TimeClamper::TimeClamper() {
 
 double TimeClamper::ClampTimeResolution(double time_seconds) const {
   DCHECK_GE(time_seconds, 0);
-  double clamped_time =
-      floor(time_seconds / kResolutionSeconds) * kResolutionSeconds;
+  double interval = floor(time_seconds / kResolutionSeconds);
+  double clamped_time = interval * kResolutionSeconds;
   double tick_threshold = ThresholdFor(clamped_time);
 
   if (time_seconds >= tick_threshold)
-    return clamped_time + kResolutionSeconds;
+    return (interval + 1) * kResolutionSeconds;
   return clamped_time;
 }
 
diff --git a/chromium/third_party/WebKit/Source/platform/TimeClamperTest.cpp b/chromium/third_party/WebKit/Source/platform/TimeClamperTest.cpp
index 04e4622c152..b8d1e59c0aa 100644
--- a/chromium/third_party/WebKit/Source/platform/TimeClamperTest.cpp
+++ b/chromium/third_party/WebKit/Source/platform/TimeClamperTest.cpp
@@ -27,6 +27,7 @@ TEST(TimeClamperTest, TimeStampsIncreaseByFixedAmount) {
        time_seconds += kInterval * 0.1) {
     double clamped_time = clamper.ClampTimeResolution(time_seconds);
     double delta = clamped_time - prev;
+    ASSERT_GE(delta, 0);
     if (delta > kEpsilon) {
       ASSERT_TRUE(std::fabs(delta - kInterval) < kEpsilon);
       prev = clamped_time;