sync: Define histogram ints for model types

sync: Define histogram ints for model types

The common practice of using enums directly when counting histograms
works only if the enums' values never change.  Unfortunately, we insert
into the middle of the list of ModelTypes all the time.

Rather than change the way we manage the list of model type enums, we
decided to separate the model types values from those used when
reporting histograms.  This change defines a mapping from model types to
integers.  These integers should be kept in sync with integer to label
mapping defined in histograms.xml.

This mapping is often similar to, but does not necessarily match, the
int values of the enums.

BUG=190015
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=188314

[rlarocque, 2013-03-14 23:34:03]

Here's the fix we talked about.  Please review.

I'm not sure how to prevent people from directly specifying the ModelType in
newly added histograms.  I'm going to add some text to histograms.xml to
discourage that practice, but I worry that most people won't look at that file
until their histograms are already checked in.  Suggestions welcome.

[Nicolas Zea, 2013-03-14 23:43:09]

LGTM

https://codereview.chromium.org/12820010/diff/1/sync/internal_api/public/base...
File sync/internal_api/public/base/model_type.h (right):

https://codereview.chromium.org/12820010/diff/1/sync/internal_api/public/base...
sync/internal_api/public/base/model_type.h:52: // WARNING: Modifying the order
of these types or inserting a new type above
You can remove this comment now right?

[rlarocque, 2013-03-15 00:31:09]

https://codereview.chromium.org/12820010/diff/1/sync/internal_api/public/base...
File sync/internal_api/public/base/model_type.h (right):

https://codereview.chromium.org/12820010/diff/1/sync/internal_api/public/base...
sync/internal_api/public/base/model_type.h:52: // WARNING: Modifying the order
of these types or inserting a new type above
On 2013/03/14 23:43:09, Nicolas Zea wrote:
> You can remove this comment now right?

Yep, good point.  Removed.

[commit-bot: I haz the power, 2013-03-15 00:34:43]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/rlarocque@chromium.org/12820010/3001

[commit-bot: I haz the power, 2013-03-15 09:13:57]

Change committed as 188314