Fix integer overflow if microseconds is too small

microseconds - kMicrosecondsPerMillisecond could cause an overflow if
microseconds is too small (close to INT64_MIN)

Bug: 1065504
Change-Id: I827dc3479eabbf1be5fdf964dbcb34bc48d7f401
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2129788Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Qijiang Fan <fqj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754979}

base/time/time.cc

@@ -402,8 +402,7 @@ int64_t Time::ToRoundedDownMillisecondsSinceUnixEpoch() const {
     // In this case, rounding towards -infinity means rounding towards 0.
     return microseconds / kMicrosecondsPerMillisecond;
   } else {
-    return (microseconds - kMicrosecondsPerMillisecond + 1) /
-           kMicrosecondsPerMillisecond;
+    return (microseconds + 1) / kMicrosecondsPerMillisecond - 1;
   }
 }
 

base/time/time_exploded_ios.cc

@@ -106,9 +106,8 @@ void Time::Explode(bool is_local, Exploded* exploded) const {
   // Calculate milliseconds ourselves, since we rounded the |seconds|, making
   // sure to round towards -infinity.
   exploded->millisecond =
-      (microsecond >= 0) ? microsecond / kMicrosecondsPerMillisecond :
-                           (microsecond - kMicrosecondsPerMillisecond + 1) /
-                               kMicrosecondsPerMillisecond;
+      (microsecond >= 0) ? microsecond / kMicrosecondsPerMillisecond
+                         : ((microsecond + 1) / kMicrosecondsPerMillisecond - 1)
 }
 
 }  // namespace base

base/time/time_exploded_posix.cc

@@ -134,8 +134,7 @@ void Time::Explode(bool is_local, Exploded* exploded) const {
     millisecond = milliseconds % kMillisecondsPerSecond;
   } else {
     // Round these *down* (towards -infinity).
-    seconds =
-        (milliseconds - kMillisecondsPerSecond + 1) / kMillisecondsPerSecond;
+    seconds = (milliseconds + 1) / kMillisecondsPerSecond - 1;
     // Make this nonnegative (and between 0 and 999 inclusive).
     millisecond = milliseconds % kMillisecondsPerSecond;
     if (millisecond < 0)