[BlobStorage] BlobMemoryController & tests

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 b7a62beac2293ee572cd2b68cc4027b3b675d700..bb3ea63ab553a7e0d024fd964594e65a688a6528 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -60261,6 +60261,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Storage.Blob.CreateDirectroyResult" enum="PlatformFileError">
+  <owner>dmurph@chromium.org</owner>
+  <summary>
+    Recorded when we create the blob storage directory for  the blob storage

[pwnall, 2016/10/18 23:20:13]

double space after "for"?

[dmurph, 2016/10/20 00:09:48]

Done.

+    system. This lazily happens when we need to save blob data to files. This

[Mark P, 2016/10/18 23:30:49]

The "this" as the beginning of the third sentence refers to saving blob data,
right?  (as opposed to the "this" as the beginning of the second sentence which
refers to create a directory)

[Mark P, 2016/10/18 23:30:49]

What does "lazily" add in this context?
Does it simply mean "... if the storage directory does not exist"?  If so,
please say that instead.
If lazily has something to do with asynchronous processing (we do it on a delay,
or when the CPU is idle, or something), please explain.

[dmurph, 2016/10/20 00:09:48]

Done.

[dmurph, 2016/10/20 00:09:48]

Clarified, it happens when we need to store blob data to files (on demand) if it
doesn't exist.

+    occurs when either we have a new blob that's larger than our memory limit,
+    or we're approaching our in-memory limit for blob storage.
+  </summary>
+</histogram>
+
 <histogram name="Storage.Blob.ExceededMemory" enum="Boolean">
   <owner>dmurph@chromium.org</owner>
   <obsolete>
@@ -60300,6 +60310,26 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Storage.Blob.PageFileSize" units="KB">
+  <owner>dmurph@chromium.org</owner>
+  <summary>
+    Size of a page file created for old blob data by the blob storage system.

[Mark P, 2016/10/18 23:30:49]

Does "old" add anything here?  I understand the whole description if I skip over
this word but with it I'm not sure what's happening.
ditto in the below description

[dmurph, 2016/10/20 00:09:48]

Done.

+    Recorded before we create the file and write the blob items. We page blob
+    data to files when we get close to our maximum blob memory usage.
+  </summary>
+</histogram>
+
+<histogram name="Storage.Blob.PagingDisabled" enum="PlatformFileError">
+  <owner>dmurph@chromium.org</owner>
+  <summary>
+    Recorded when we disable paging blobs to file in the blob storage system.
+    This occurs when we experience a filesystem error during paging old blob
+    data to disk - when the system is close to it's in-memory limit - or

[Mark P, 2016/10/18 23:30:49]

nit: it's -> its

[dmurph, 2016/10/20 00:09:48]

Done.

+    creating files for a new blob that's initially larger than our memory limit.
+    The relevant error is recorded here.
+  </summary>
+</histogram>
+
 <histogram name="Storage.Blob.RendererFileSeekFailed" enum="Boolean">
   <owner>dmurph@chromium.org</owner>
   <summary>