Create MemoryPressureStatsCollector.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index f9b9ebdebef0cf7a01fe6844262ce007491af345..23fecfac22b6b02570d97d3bdf3ebd7d2cafb67b 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -3801,8 +3801,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
-<histogram name="ChromeOS.MemoryPressureLevel"
-    enum="ChromeOSMemoryPressureLevel">
+<histogram name="ChromeOS.MemoryPressureLevel" enum="MemoryPressureLevel">
   <owner>xdai@chromium.org</owner>
   <summary>
     The memory pressure level in ChromeOS, which is recorded periodically (once
@@ -18097,12 +18096,17 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 <histogram name="Memory.PressureLevel" enum="MemoryPressureLevel">
   <owner>chrisha@chromium.org</owner>
   <summary>
-    The memory pressure level, which is recorded periodically (once per second).
-    This is used to show the relative time that systems spend in each of the
-    memory pressure states.
+    The memory pressure level, which is recorded periodically. This shows the
+    cumulative number of seconds that systems spend in each of the memory
+    pressure states.
   </summary>
 </histogram>
 
+<histogram name="Memory.PressureLevelChanges" enum="MemoryPressureLevelChanges">
+  <owner>chrisha@chromium.org</owner>
+  <summary>The number of pressure level state changes.</summary>

[rkaplow, 2015/09/03 22:15:07]

maybe make it extra explicit .. for each possible pairwise state change.
Currently it seems like it could just the the aggregate.

[chrisha, 2015/09/08 15:47:14]

Done.

+</histogram>
+
 <histogram name="Memory.ProcessCount">
   <owner>hajimehoshi@chromium.org</owner>
   <owner>kenjibaheux@google.com</owner>
@@ -54084,12 +54088,6 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="2" label="Periodic"/>
 </enum>
 
-<enum name="ChromeOSMemoryPressureLevel" type="int">
-  <int value="0" label="No memory pressure"/>
-  <int value="1" label="Medium memory pressure"/>
-  <int value="2" label="High memory pressure"/>
-</enum>
-
 <enum name="ChromeOSPlatformVerificationExpiryStatus" type="int">
   <summary>
     Possible results of a platform verification expiry check. See
@@ -65067,6 +65065,15 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="2" label="Critical memory pressure"/>
 </enum>
 
+<enum name="MemoryPressureLevelChanges" type="int">
+  <int value="0" label="No pressure to moderate pressure"/>
+  <int value="1" label="No pressure to critical pressure"/>
+  <int value="2" label="Moderate pressure to critical pressure"/>
+  <int value="3" label="Critical pressure to moderate pressure"/>
+  <int value="4" label="Critical pressure to no pressure"/>
+  <int value="5" label="Moderate pressure to no pressure"/>
+</enum>
+
 <enum name="MetadataReadResult" type="int">
   <int value="0" label="Success"/>
   <int value="1" label="Open failure"/>