Rename RuntimeContext to XWalkBrowserContext

Since it's a browser context actually.
gyp: Cleanup Aura-related files and dependencies.

* Stop manually excluding _aura.cc files from the build, this is done
  automatically by Chromium's filename_rules.gypi. Ditto for files
  ending in _views.cc.

* Remove the dependency on the "aura_test_support" and "wm" targets. Not
  only does depending on a test target even when no tests are being
  bring a ton of additional dependencies, but neither targets seem to be
  needed at all, and were probably only needed in an earlier version of
  commit 336c03a ("runtime: Add aura-only native support").
Merge pull request #2673 from rakuco/cleanup-aura-gyp-deps

gyp: Cleanup Aura-related files and dependencies.
Merge pull request #2671 from pozdnyakov/rename_RuntimeContext

Rename RuntimeContext to XWalkBrowserContext