base: Introduce Process::GetPidInNamespace().

On Chrome OS, we run several daemons as separate PID
namespaces. This function converts system PID to namespace PID.

BUG=567632
TEST=GetPidInNamespace() returns correct PID for proc in ns

Review URL: https://codereview.chromium.org/1487273004

Cr-Commit-Position: refs/heads/master@{#363963}
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 52e3b62..9a7f4c34 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -164,6 +164,7 @@
       ),
       True,
       (
+        r"^base[\\\/]process[\\\/]process_linux\.cc$",
         r"^base[\\\/]process[\\\/]process_metrics_linux\.cc$",
         r"^chrome[\\\/]browser[\\\/]chromeos[\\\/]boot_times_recorder\.cc$",
         r"^chrome[\\\/]browser[\\\/]chromeos[\\\/]"