Added GetCpuTime to base/ to get total CPU time consumed by process for perf tests.

webrtc/base/BUILD.gn

Index: webrtc/base/BUILD.gn
diff --git a/webrtc/base/BUILD.gn b/webrtc/base/BUILD.gn
index 83e36cb59df7388c6c73c9c83f4534a757df5b13..08799915b882a436bc4d290636b0f7605989694f 100644
--- a/webrtc/base/BUILD.gn
+++ b/webrtc/base/BUILD.gn
@@ -684,6 +684,8 @@ if (rtc_include_tests) {
     sources = [
       # Also use this as a convenient dumping ground for misc files that are
       # included by multiple targets below.
+      "cpu_time.cc",
+      "cpu_time.h",
       "fakeclock.cc",
       "fakeclock.h",
       "fakenetwork.h",
@@ -765,6 +767,7 @@ if (rtc_include_tests) {
       "bytebuffer_unittest.cc",
       "byteorder_unittest.cc",
       "copyonwritebuffer_unittest.cc",
+      "cpu_time_unittest.cc",
       "criticalsection_unittest.cc",
       "event_tracer_unittest.cc",
       "event_unittest.cc",