[jumbo] last_system_time_ is only used on mac + win

Avoid "error: private field 'last_system_time_' is not used" warnings
which are promoted to errors.  This was somehow triggered by this CL:
https://chromium-review.googlesource.com/973943
(But I'm not sure how- the CL did not remove any direct uses of that
variable.)

Speculative fix applied to the openbsd and freebsd implementations.

TBR=dcheng@chromium.org

Bug: 819289
Change-Id: I862d9c22a8e181ac5dd3f1a644432e5570419a0b
Reviewed-on: https://chromium-review.googlesource.com/979432Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Mostyn Bramley-Moore <mostynb@vewd.com>
Commit-Queue: Mostyn Bramley-Moore <mostynb@vewd.com>
Cr-Commit-Position: refs/heads/master@{#545689}

base/process/process_metrics.h

@@ -219,7 +219,9 @@ class BASE_EXPORT ProcessMetrics {
   // Used to store the previous times and CPU usage counts so we can
   // compute the CPU usage between calls.
   TimeTicks last_cpu_time_;
+#if defined(OS_WIN) || defined(OS_MACOSX)
   int64_t last_system_time_;
+#endif
 
 #if defined(OS_MACOSX) || defined(OS_LINUX) || defined(OS_AIX)
   // Same thing for idle wakeups.

base/process/process_metrics_freebsd.cc

@@ -17,7 +17,6 @@ namespace base {
 
 ProcessMetrics::ProcessMetrics(ProcessHandle process)
     : process_(process),
-      last_system_time_(0),
       last_cpu_(0) {}
 
 // static

base/process/process_metrics_linux.cc

@@ -358,7 +358,6 @@ int ProcessMetrics::GetOpenFdSoftLimit() const {
 
 ProcessMetrics::ProcessMetrics(ProcessHandle process)
     : process_(process),
-      last_system_time_(0),
 #if defined(OS_LINUX) || defined(OS_AIX)
       last_absolute_idle_wakeups_(0),
 #endif

base/process/process_metrics_openbsd.cc

@@ -70,7 +70,6 @@ double ProcessMetrics::GetPlatformIndependentCPUUsage() {
 
 ProcessMetrics::ProcessMetrics(ProcessHandle process)
     : process_(process),
-      last_system_time_(0),
       last_cpu_(0) {}
 
 size_t GetSystemCommitCharge() {