Add support for repeated keys in TaskRequest.

Add support for repeated keys in TaskRequest.

- Causes all properties_hash to change, so task deduplication is temporarily
  affected.
- While the internals changed, task_queues.hash_dimensions() is unaffected, so
  the task queues will continue working as before.
- Most of the changes are in unit tests due to the way dimensions specification
  changed.
- One local smoke test was updated to specify duplicate key to confirm it works
  end-to-end.
- Explicitly deny ':' in key for request earlier in the pipeline due to the
  internal encoding. It makes sense because this is also how tags are encoded.
- This change is only backward compatible up to the gap commit.

R=vadimsh@chromium.org
BUG=728124

[M-A Ruel, 2017-06-07 20:30:37]

Description was changed from

==========
Add support for repeated keys in TaskRequest.

This change causes all properties_hash to change, so task deduplication is
temporarily affected.

While the internals changed, task_queues.hash_dimensions() is unaffected, so the
task queues will continue working as before.

Most of the changes are in unit tests due to the way dimensions specification
changed.

R=vadimsh@chromium.org
BUG=728124
==========

to

==========
Add support for repeated keys in TaskRequest.

- Causes all properties_hash to change, so task deduplication is temporarily
  affected.
- While the internals changed, task_queues.hash_dimensions() is unaffected, so
  the task queues will continue working as before.
- Most of the changes are in unit tests due to the way dimensions specification
  changed.
- One local smoke test was updated to specify duplicate key to confirm it works
  end-to-end.
- Explicitly deny ':' in key for request earlier in the pipeline due to the
  internal encoding. It makes sense because this is also how tags are encoded.
- This change is not backward compatible.

R=vadimsh@chromium.org
BUG=728124
==========

[M-A Ruel, 2017-06-07 20:31:29]

Patchset #1 (id:1) has been deleted

[M-A Ruel, 2017-06-07 21:05:05]

Description was changed from

==========
Add support for repeated keys in TaskRequest.

- Causes all properties_hash to change, so task deduplication is temporarily
  affected.
- While the internals changed, task_queues.hash_dimensions() is unaffected, so
  the task queues will continue working as before.
- Most of the changes are in unit tests due to the way dimensions specification
  changed.
- One local smoke test was updated to specify duplicate key to confirm it works
  end-to-end.
- Explicitly deny ':' in key for request earlier in the pipeline due to the
  internal encoding. It makes sense because this is also how tags are encoded.
- This change is not backward compatible.

R=vadimsh@chromium.org
BUG=728124
==========

to

==========
Add support for repeated keys in TaskRequest.

- Causes all properties_hash to change, so task deduplication is temporarily
  affected.
- While the internals changed, task_queues.hash_dimensions() is unaffected, so
  the task queues will continue working as before.
- Most of the changes are in unit tests due to the way dimensions specification
  changed.
- One local smoke test was updated to specify duplicate key to confirm it works
  end-to-end.
- Explicitly deny ':' in key for request earlier in the pipeline due to the
  internal encoding. It makes sense because this is also how tags are encoded.
- This change is only backward compatible up to the gap commit.

R=vadimsh@chromium.org
BUG=728124
==========

[M-A Ruel, 2017-06-07 21:05:15]

Patchset #1 (id:20001) has been deleted

[M-A Ruel, 2017-06-07 21:16:40]

Patchset #1 (id:40001) has been deleted

[M-A Ruel, 2017-06-07 21:18:48]

I plan to deploy https://codereview.chromium.org/2928823002 first everywhere so
if any issue come up, reverting should be easy (unless I made an error).

In practice this change doesn't concerns me since it's fairly simple but I
prefer to take precaution, hence the previous CL.