[Telemetry] Introduce SummarizableValue.

[Telemetry] Introduce SummarizableValue.

This is a refresh of crrev.com/809393002, wherein
improvement_direction was added to Value.

This CL pulls out only the Telemetry-side changes
in an attempt to make this CL a bit smaller; the
next change will be to update Telemetry clients to
use this new field.

BUG=459450
Committed: https://crrev.com/5ad6f2b91267a69d173713e1607774cac8bbf58a
Cr-Commit-Position: refs/heads/master@{#346410}

[eakuefner, 2015-08-27 18:28:14]

eakuefner@chromium.org changed reviewers:
+ aiolos@chromium.org, nednguyen@google.com, sullivan@chromium.org

[eakuefner, 2015-08-27 18:33:47]

PTAL. This is a pretty direct refresh of Annie's CL, which at the time was
pretty much ready to go. Some notes for reviewers:

1. There's one TODO in SummarizableValue to enable the assert that checks that
all SummarizableValues have an indicated improvement_direction. All Telemetry
unittests pass with the assert enabled, but we need to update client code's
AddValue callsites, which I want to do in a forthcoming CL since this one is
already so large.
2. I've opened crbug.com/483646 about fixing serialization/deserialization for
values, which links to a short design doc I wrote. You may notice that the tests
having to do with AsDict/FromDict are a little messy, but I don't think that the
tests Annie originally added are undertesting; they're just unavoidably in the
wrong place (subclasses) because of the design issue that I'm going to fix.
3. I've opened crbug.com/525688 about rewriting Summary from scratch. You would
expect that a value named Summarizable would receive special treatment in
Summary, which is currently hard or impossible based on the fact that Summary is
spaghetti. This is what Annie's CL stalled on previously, but I don't think
keeping summarization as-is for now blocks this code.

[eakuefner, 2015-08-27 18:35:56]

The CQ bit was checked by eakuefner@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-08-27 18:37:05]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1313243003/60001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1313243003/60001

[commit-bot: I haz the power, 2015-08-27 18:39:36]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-08-27 18:39:36]

Dry run: Try jobs failed on following builders:
  linux_chromium_compile_dbg_32_ng on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_...)
  ios_dbg_simulator_ninja on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_dbg_simulator...)
  ios_rel_device_ninja on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios_rel_device_ni...)
  mac_chromium_compile_dbg_ng on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_comp...)
  mac_chromium_rel_ng on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_rel_...)

[eakuefner, 2015-08-27 20:44:27]

The CQ bit was checked by eakuefner@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-08-27 20:44:42]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1313243003/80001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1313243003/80001

[commit-bot: I haz the power, 2015-08-27 22:04:25]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-08-27 22:04:26]

Dry run: This issue passed the CQ dry run.

[sullivan, 2015-08-31 13:11:50]

lgtm

[nednguyen, 2015-08-31 16:11:40]

lgtm
As discussed offline, we should gravitate toward moving the public APIs in
package into the package/__init__.py.

[eakuefner, 2015-08-31 16:30:55]

On 2015/08/31 at 16:11:40, nednguyen wrote:
> lgtm
> As discussed offline, we should gravitate toward moving the public APIs in
package into the package/__init__.py.

Eventually, we may also consider migrating improvement_direction into a "unit"
module, so that we would have e.g. unit.IMPROVEMENT_DIRECTION_DOWN and
unit.IMPROVEMENT_DIRECTION_UP or something.

[eakuefner, 2015-08-31 16:31:07]

The CQ bit was checked by eakuefner@chromium.org

[commit-bot: I haz the power, 2015-08-31 16:31:52]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1313243003/80001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1313243003/80001

[commit-bot: I haz the power, 2015-08-31 17:38:15]

Committed patchset #5 (id:80001)

[commit-bot: I haz the power, 2015-08-31 17:38:50]

Patchset 5 (id:??) landed as
https://crrev.com/5ad6f2b91267a69d173713e1607774cac8bbf58a
Cr-Commit-Position: refs/heads/master@{#346410}