Optimize memory use of registered histograms.

Optimize memory use of registered histograms.

Prior to this change, all histogram names would be stored
twice - once in each Histogram object and another time as
the key of the histograms map. This change eliminates the
second copy by making the map key be a reference to the
name string within the Histogram object.

In my local testing, this saved about 35k of memory in the
browser process and around 2k-4k in each sub-process.

BUG=500619
Committed: https://crrev.com/33d9fd6321af0d2905adb9f5168d853e3b45248c
Cr-Commit-Position: refs/heads/master@{#334481}

[Alexei Svitkine (slow), 2015-06-15 16:51:46]

Patchset #1 (id:1) has been deleted

[Alexei Svitkine (slow), 2015-06-15 17:19:46]

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

[Ilya Sherman, 2015-06-15 21:50:14]

LGTM.  Nice find!

Also, wow, 35k of memory just for a single copy of the histogram names?  How
much memory are we spending on histograms in total?

[Alexei Svitkine (slow), 2015-06-15 21:53:27]

I did measurement that too - and it was about 200k in a clean-ish profile (where
this fix saved 17k). So likely 500k-ish in a busier one.

We definitely have room for improvements. For example, around 40% of those 200k
were histogram with <= 1 sample, yet we were still storing all their buckets.

[Alexei Svitkine (slow), 2015-06-15 21:54:06]

The CQ bit was checked by asvitkine@chromium.org

[commit-bot: I haz the power, 2015-06-15 21:55:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1181293005/60001

[commit-bot: I haz the power, 2015-06-15 22:36:27]

Committed patchset #3 (id:60001)

[commit-bot: I haz the power, 2015-06-15 22:37:24]

Patchset 3 (id:??) landed as
https://crrev.com/33d9fd6321af0d2905adb9f5168d853e3b45248c
Cr-Commit-Position: refs/heads/master@{#334481}