base: Introduce SharedMemoryTracker for POSIX (but not macOS)

This CL introduces SharedMemoryTracker to measure memory usage of
SharedMemoy correctly and report them to memory-infra.
SharedMemoryTracker records the memory usage when mmap-ing and
when unmmap-ing, and reports them when OnMemoryDump is called.
This enables to help us know SharedMemory usage correctly.

This is a part of a CL (https://codereview.chromium.org/2535213002/) for
SharedMemory usage measuring.

BUG=604726

Review-Url: https://codereview.chromium.org/2654073002
Cr-Commit-Position: refs/heads/master@{#455406}
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index e56d4b3..96f8de21 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -185,6 +185,7 @@
       ),
       True,
       (
+        r"^base[\\\/]memory[\\\/]shared_memory_posix\.cc$",
         r"^base[\\\/]process[\\\/]process_linux\.cc$",
         r"^base[\\\/]process[\\\/]process_metrics_linux\.cc$",
         r"^chrome[\\\/]browser[\\\/]chromeos[\\\/]boot_times_recorder\.cc$",