Add a metric recording impressions.

Also consider adding a metric recording age of the clipboard, a la
https://cs.chromium.org/chromium/src/ios/chrome/browser/ui/omnibox/omnibox_popup_view_ios.mm?type=cs&q=GetClipboardContentAge()&l=294

Plus migrate MobileOmniboxClipboardChanged user action on both iOS and Android to MobileClipboardChanged.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ab3761497a481d8b747d3e7342e8fbe8768fef4b

commit ab3761497a481d8b747d3e7342e8fbe8768fef4b
Author: mpearson <mpearson@chromium.org>
Date: Fri Mar 31 17:29:51 2017

Omnibox Metrics - Record Clipboard URL Impressions

Not tested, as this provider is only implemented on an iOS device and I don't have one.  I'm pretty sure this is straightforward enough to work right. :-)  [famous last words]

BUG= 704715 

Review-Url: https://codereview.chromium.org/2782393003
Cr-Commit-Position: refs/heads/master@{#461161}

[modify] https://crrev.com/ab3761497a481d8b747d3e7342e8fbe8768fef4b/components/omnibox/browser/clipboard_url_provider.cc
[modify] https://crrev.com/ab3761497a481d8b747d3e7342e8fbe8768fef4b/tools/metrics/histograms/histograms.xml

In addition to comment #1 about adding an age of the clipboard at time of use, it would help to add an age at the time of serving the impression.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4a49ce9b7bfe546a90e6f3ccc2b6c34de764463c

commit 4a49ce9b7bfe546a90e6f3ccc2b6c34de764463c
Author: mpearson <mpearson@chromium.org>
Date: Thu Apr 06 09:29:58 2017

Omnibox Metrics - Rename MobileOmniboxClipboardChanged

to MobileClipboardChanged.  (Mark the old one as obsolete; make myself one of the owners of the new one.)

After all, the code to watch for this has nothing directly to do with
the omnibox or an "omnibox clipboard".

BUG= 704715 

Review-Url: https://codereview.chromium.org/2800483002
Cr-Commit-Position: refs/heads/master@{#462402}

[modify] https://crrev.com/4a49ce9b7bfe546a90e6f3ccc2b6c34de764463c/components/open_from_clipboard/clipboard_recent_content_ios.mm
[modify] https://crrev.com/4a49ce9b7bfe546a90e6f3ccc2b6c34de764463c/tools/metrics/actions/actions.xml
[modify] https://crrev.com/4a49ce9b7bfe546a90e6f3ccc2b6c34de764463c/ui/android/java/src/org/chromium/ui/base/Clipboard.java

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d3dcdb05fa128fc8958a5772734847198324e25b

commit d3dcdb05fa128fc8958a5772734847198324e25b
Author: mpearson <mpearson@chromium.org>
Date: Sat Apr 08 01:11:03 2017

Omnibox Metrics - Record Age of Clipboard Suggestions Shown

Also move histogram MobileOmnibox.PressedClipboardSuggestionAge from the internal google repository to histograms.xml.  In the process, expand the histogram description and revise the owners of this histogram.

Tested using about:histograms.

BUG= 704715 

Review-Url: https://codereview.chromium.org/2792173004
Cr-Commit-Position: refs/heads/master@{#463093}

[modify] https://crrev.com/d3dcdb05fa128fc8958a5772734847198324e25b/components/omnibox/browser/clipboard_url_provider.cc
[modify] https://crrev.com/d3dcdb05fa128fc8958a5772734847198324e25b/tools/metrics/histograms/histograms.xml