Add histograms to record the time since tab was last active.

Add histograms to record the time since tab was last active.

Adds histograms to record the time since a tab was active when:
 - switching to the tab
 - switching to an evicted tab
 - protecting a tab on memory warning
 - evicting a tab on memory warning

Also adds a histogram to show how often users switch to
an evicted tab that was never active.

BUG=385111
R=asvitkine@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=284061

[Alexei Svitkine (slow), 2014-06-20 14:48:56]

For all of these, could you mention that these are specific to iOS?

https://codereview.chromium.org/340113003/diff/1/histograms/histograms.xml
File histograms/histograms.xml (right):

https://codereview.chromium.org/340113003/diff/1/histograms/histograms.xml#ne...
histograms/histograms.xml:30403: +    was active.
Do you mean "the since it was made inactive"? Or "the time since it was
activated the previous time it was active"? Please clarify. Same for the one
above.

[lliabraa, 2014-06-26 16:46:29]

https://codereview.chromium.org/340113003/diff/1/histograms/histograms.xml
File histograms/histograms.xml (right):

https://codereview.chromium.org/340113003/diff/1/histograms/histograms.xml#ne...
histograms/histograms.xml:30403: +    was active.
On 2014/06/20 14:48:56, Alexei Svitkine wrote:
> Do you mean "the since it was made inactive"? Or "the time since it was
> activated the previous time it was active"? Please clarify. Same for the one
> above.

Done.

[Alexei Svitkine (slow), 2014-06-26 17:00:43]

lgtm % comment and other cl landing

https://codereview.chromium.org/340113003/diff/20001/histograms/histograms.xml
File histograms/histograms.xml (right):

https://codereview.chromium.org/340113003/diff/20001/histograms/histograms.xm...
histograms/histograms.xml:30277: +    tab had ever been active.
What's the case where it wouldn't have ever been active? Is it from session
restore? (If so, might be worth mentioning that specifically in description to
make it obvious.)

[lliabraa, 2014-07-09 18:55:47]

https://chromiumcodereview.appspot.com/340113003/diff/20001/histograms/histog...
File histograms/histograms.xml (right):

https://chromiumcodereview.appspot.com/340113003/diff/20001/histograms/histog...
histograms/histograms.xml:30277: +    tab had ever been active.
On 2014/06/26 17:00:43, Alexei Svitkine wrote:
> What's the case where it wouldn't have ever been active? Is it from session
> restore? (If so, might be worth mentioning that specifically in description to
> make it obvious.)

Done.

[lliabraa, 2014-07-18 12:04:31]

Committed patchset #3 manually as r284061 (presubmit successful).