This CL updates various files in the includes directory to ensure that (1) they do
not depend on headers in /src and (2) that they minimize their dependence on external
headers.
To ensure that we don't regress this behavior a new build target has been added to
build a single cpp file that contains all* public includes and is compiled with
only those directories in the include path.
* The exception is those includes that depend on OS specific headers
BUG=skia:2941
NOTRY=true
Review URL: https://codereview.chromium.org/721903002
