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Add a skeleton build file for content/common that lists the basic deps, and processes the sources. This can keep track of the deps that have been converted and allows easy testing of the .gypi reading.

Changes content_common.gypi to separate out the public and private sources. The .gypi combines them both into the 'sources' so current users of this file should not be affected. This allows the GN build to declare the public API.

Enhance the gypi-to-gn script to strip conditions and collapse variables to allow the content .gypis to be read in.

Adds some simple BUILD.gn files for dependencies of content/common

Fix typo in accessibility.gyp that I discovered when writing the GN build.

R=jam@chromium.org

Review URL: https://codereview.chromium.org/287233002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@271767 0039d316-1c4b-4281-b951-d872f2087c98