Added include for size_t to ContainerMask.h

[Teemperor]

We use size_t in this header but never include a header that defines
size_t. Let's just include to make this header standalone.

[Teemperor]

Part of the work going on regarding the C++ modules migration of CMSSW (tracked as issue #15248). This PR is not meant to refactor things, but just to make these headers compile.

[cmsbuild]

The code-checks are being triggered in jenkins.

[cmsbuild]

+code-checks

Logs: https://cmssdt.cern.ch/SDT/code-checks/PR-21372/2073

[cmsbuild]

A new Pull Request was created by @Teemperor (Raphael Isemann) for master.

It involves the following packages:

DataFormats/Common

@cmsbuild, @smuzaffar, @Dr15Jones can you please review it and eventually sign? Thanks.
@makortel, @ebrondol, @rovere, @wddgit this is something you requested to watch as well.
@davidlange6, @slava77 you are the release manager for this.

cms-bot commands are listed here

[davidlange6]

please test

[cmsbuild]

The tests are being triggered in jenkins.
https://cmssdt.cern.ch/jenkins/job/ib-any-integration/24543/console Started: 2017/11/20 15:49

[cmsbuild]

+1
Tested at: 334f286
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-21372/24543/summary.html

The following merge commits were also included on top of IB + this PR after doing git cms-merge-topic:
8ecebe6
9d70585
71a61e3
You can see more details here:
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-21372/24543/git-log-recent-commits
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-21372/24543/git-merge-result

[cmsbuild]

Comparison job queued.

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-21372/24543/summary.html

Comparison Summary:

• No significant changes to the logs found
• Reco comparison results: 0 differences found in the comparisons
• DQMHistoTests: Total files compared: 27
• DQMHistoTests: Total histograms compared: 2833444
• DQMHistoTests: Total failures: 1
• DQMHistoTests: Total nulls: 0
• DQMHistoTests: Total successes: 2833265
• DQMHistoTests: Total skipped: 178
• DQMHistoTests: Total Missing objects: 0
• DQMHistoSizes: Histogram memory added: 1.26000000001 KiB( 23 files compared)
• Checked 111 log files, 8 edm output root files, 27 DQM output files

[davidlange6]

merge