Add histograms to track age of cache entries for web fonts

Add histograms to track age of cache entries for web fonts

This CL adds UMA histograms that tracks age (time since creation) of
cache entries for web fonts. Histograms are suffixed with font names:
"roboto", "opensans", or "others".

Also, fixes a bug that DiskCache.ReuseCount.Evict was recording
evictions of non-webfont entries.

BUG=379670
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=276599

[rvargas (doing something else), 2014-06-10 23:15:34]

lgtm after fixing the key.

https://codereview.chromium.org/323013003/diff/1/net/disk_cache/blockfile/web...
File net/disk_cache/blockfile/webfonts_histogram.cc (right):

https://codereview.chromium.org/323013003/diff/1/net/disk_cache/blockfile/web...
net/disk_cache/blockfile/webfonts_histogram.cc:82: const char* label =
HistogramLabel(info->key);
I just realized that this is wrong. This should not be accessing the key field
directly, it should be calling entry->GetKey()

[Ilya Sherman, 2014-06-10 23:29:23]

+Jesse for histograms review

[Kunihiko Sakamoto, 2014-06-11 01:50:10]

https://codereview.chromium.org/323013003/diff/1/net/disk_cache/blockfile/web...
File net/disk_cache/blockfile/webfonts_histogram.cc (right):

https://codereview.chromium.org/323013003/diff/1/net/disk_cache/blockfile/web...
net/disk_cache/blockfile/webfonts_histogram.cc:82: const char* label =
HistogramLabel(info->key);
On 2014/06/10 23:15:33, rvargas wrote:
> I just realized that this is wrong. This should not be accessing the key field
> directly, it should be calling entry->GetKey()

Done.

[jwd, 2014-06-11 17:22:32]

lgtm for histograms

[Ilya Sherman, 2014-06-11 17:24:20]

histograms LGTM2

[Kunihiko Sakamoto, 2014-06-12 01:00:41]

The CQ bit was checked by ksakamoto@chromium.org

[commit-bot: I haz the power, 2014-06-12 01:01:36]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/ksakamoto@chromium.org/323013003/20001

[commit-bot: I haz the power, 2014-06-12 07:06:12]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  mac_chromium_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/mac_chromium_rel/buil...)

[commit-bot: I haz the power, 2014-06-12 09:27:54]

Change committed as 276599