Add GN build file for components/storage_monitor

A line-by-line conversion of the existing GYP configs.

R=ajwong
TBR=gbillock

Review URL: https://codereview.chromium.org/387773003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@283007 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/components/storage_monitor.gypi b/components/storage_monitor.gypi
index e43cf4c4..e522e8df 100644
--- a/components/storage_monitor.gypi
+++ b/components/storage_monitor.gypi
@@ -5,6 +5,7 @@
 {
   'targets': [
     {
+      # GN version: //components/storage_monitor
       'target_name': 'storage_monitor',
       'type': 'static_library',
       'include_dirs': [
@@ -88,6 +89,7 @@
       ],
     },
     {
+      # GN version: //components/storage_monitor:test_support
       'target_name': 'storage_monitor_test_support',
       'type': 'static_library',
       'include_dirs': [