Add aggregated memory histograms.

Add aggregated memory histograms.

This introduces V8.MemoryHeapCommitted and V8.MemoryHeapUsed histograms.

In contrast to the existing memory histograms, the new histograms are uniform in time, i.e. their samples happen at regular time intervals. The --histogram-interval specifies the length of the interval.

We implement this by linearly interpolating memory stats between GC and idle notification events.

BUG=chromium:485472
LOG=NO

Committed: https://crrev.com/d77839fd0182d01e6d7864f0312d2b194deea2f0
Cr-Commit-Position: refs/heads/master@{#28292}

[ulan, 2015-05-05 10:31:29]

ulan@chromium.org changed reviewers:
+ jochen@chromium.org

[ulan, 2015-05-05 10:31:30]

ptal

[ulan, 2015-05-05 10:34:54]

Hannes: fyi

[jochen (gone - plz use gerrit), 2015-05-05 14:34:23]

lgtm

please link to the design doc, and include some explanation on how the average
is computed

[ulan, 2015-05-07 09:30:09]

The CQ bit was checked by ulan@chromium.org to run a CQ dry run

[ulan, 2015-05-07 09:30:09]

The patchset sent to the CQ was uploaded after l-g-t-m from jochen@chromium.org
Link to the patchset: https://codereview.chromium.org/1125683004/#ps80001
(title: "Add comments")

[commit-bot: I haz the power, 2015-05-07 09:30:24]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1125683004/80001

[commit-bot: I haz the power, 2015-05-07 09:57:13]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-05-07 09:57:14]

Dry run: This issue passed the CQ dry run.

[ulan, 2015-05-07 10:01:43]

The CQ bit was checked by ulan@chromium.org

[commit-bot: I haz the power, 2015-05-07 10:01:54]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1125683004/80001

[commit-bot: I haz the power, 2015-05-07 10:03:39]

Committed patchset #5 (id:80001)

[commit-bot: I haz the power, 2015-05-07 10:03:54]

Patchset 5 (id:??) landed as
https://crrev.com/d77839fd0182d01e6d7864f0312d2b194deea2f0
Cr-Commit-Position: refs/heads/master@{#28292}

[ulan, 2015-05-07 10:45:41]

> please link to the design doc, and include some explanation on how the average
is computed
Added bug ID and linked to the doc there.
Added comments about invariants that hold for the average computation.

[Alexei Svitkine (slow), 2016-06-28 16:54:39]

asvitkine@chromium.org changed reviewers:
+ asvitkine@chromium.org

[Alexei Svitkine (slow), 2016-06-28 16:54:39]

https://codereview.chromium.org/1125683004/diff/80001/src/counters.h
File src/counters.h (right):

https://codereview.chromium.org/1125683004/diff/80001/src/counters.h#newcode540
src/counters.h:540: HM(heap_sample_maximum_committed,
V8.MemoryHeapSampleMaximumCommitted)
Hi, this histogram doesn't appear to be in histograms.xml. Can it be added
there?

Additionally, since it's not in the XML, it's data is not visible on UMA
dashboards, which likely means it's not used. If so, please consider removing it
- as it's currently one of the top unused histograms that's wasting users'
bandwidth and memory. Thanks!