Reland: content_{renderer,tests}.gypi cleanup of plugin+webrtc sources

Reland: content_{renderer,tests}.gypi cleanup of plugin+webrtc sources

Relanding after adding the missing BUILD.gn configuration.

TBR=mark@chromium.org, rationale: this patch is equivalent
to the already LGTMd plus some stale .gyp code removal.

Original description ---------------------------------------------------
content_{renderer,tests}.gypi cleanup of plugin+webrtc sources

Moving around some files after renaming made more evident
what they do.

BUG=525817, 530330
Committed: https://crrev.com/fc3c2df218caee93a913612a6ad6ef84f6169d0c
Cr-Commit-Position: refs/heads/master@{#348232}

Committed: https://crrev.com/6029f8d1cd9c3217d4640abb9071a3e880f9ca66
Cr-Commit-Position: refs/heads/master@{#348468}

[mcasas, 2015-09-11 19:07:10]

The CQ bit was checked by mcasas@chromium.org

[commit-bot: I haz the power, 2015-09-11 19:07:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1335033003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1335033003/20001

[commit-bot: I haz the power, 2015-09-11 19:07:54]

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

[commit-bot: I haz the power, 2015-09-11 19:07:54]

No L-G-T-M from a valid reviewer yet. Only full committers are accepted.
Even if an L-G-T-M may have been provided, it was from a non-committer,
_not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[mcasas, 2015-09-11 19:11:33]

The CQ bit was checked by mcasas@chromium.org

[commit-bot: I haz the power, 2015-09-11 19:12:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1335033003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1335033003/20001

[commit-bot: I haz the power, 2015-09-11 19:52:41]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-09-11 19:53:40]

Patchset 2 (id:??) landed as
https://crrev.com/6029f8d1cd9c3217d4640abb9071a3e880f9ca66
Cr-Commit-Position: refs/heads/master@{#348468}

[commit-bot: I haz the power, 2015-09-23 12:25:31]

Patchset 2 (id:??) landed as
https://crrev.com/6029f8d1cd9c3217d4640abb9071a3e880f9ca66
Cr-Commit-Position: refs/heads/master@{#348468}