Add UMA histogram for suppressed dialogs on prerender swap.

Add UMA histogram for suppressed dialogs on prerender swap.

Get statistics on how often OnCloseWebContentsDeleter would have showed a
dialog, presumably for onbeforeunload, just to see how often this actually
occurs.

BUG=304932
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=232908

[davidben, 2013-11-04 18:13:19]

We need to find a way to fix this anyway, but statistics are probably handy.

asvitkine: For histograms.xml.

[mmenke, 2013-11-04 18:42:17]

LGTM.  This could also catch dialogs shown after a link is clicked, but before
the swap occurs, right?  Not that it's terribly likely that will happen.

[Alexei Svitkine (slow), 2013-11-04 18:47:57]

https://codereview.chromium.org/58053002/diff/1/tools/metrics/histograms/hist...
File tools/metrics/histograms/histograms.xml (right):

https://codereview.chromium.org/58053002/diff/1/tools/metrics/histograms/hist...
tools/metrics/histograms/histograms.xml:13737: +<histogram
name="Prerender.TabContentsDeleterSuppressedDialog">
Please add a BooleanSuppressed enum and specify it here via the enum= attribute.

[davidben, 2013-11-04 19:07:30]

> This could also catch dialogs shown after a link is clicked, but before
> the swap occurs, right?  Not that it's terribly likely that will happen

Maybe? Most dialogs on shutdown are also suppressed on unload anyway, although
perhaps there's a race possible. I haven't looked at how that's implemented.
(beforeunload event handlers don't show a dialog directly. They return a string
for the browser to do what it wants with. Though we implement it with a
synchronous IPC and app-modal dialog.)

https://codereview.chromium.org/58053002/diff/1/tools/metrics/histograms/hist...
File tools/metrics/histograms/histograms.xml (right):

https://codereview.chromium.org/58053002/diff/1/tools/metrics/histograms/hist...
tools/metrics/histograms/histograms.xml:13737: +<histogram
name="Prerender.TabContentsDeleterSuppressedDialog">
On 2013/11/04 18:47:58, Alexei Svitkine wrote:
> Please add a BooleanSuppressed enum and specify it here via the enum=
attribute.

Done. Also added an enum for TabContentsDeleterTimeout.

[Alexei Svitkine (slow), 2013-11-04 19:14:17]

lgtm

[commit-bot: I haz the power, 2013-11-04 19:26:17]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/davidben@chromium.org/58053002/60001

[commit-bot: I haz the power, 2013-11-05 03:27:41]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/davidben@chromium.org/58053002/60001

[commit-bot: I haz the power, 2013-11-05 04:37:54]

Change committed as 232908