gn-build: Clean up GN files

Removed unnecessary library and include flags from build files. To make the
initial build outputs more similar to Android, this CL changes the usage of the
GN "source_set" idiom to "static_library". This comes at a performance loss
while keeping the builds similar until we get the whole stack to compile.

BUG=21339022

Change-Id: I46f70b3b64b2537634a1dbb517a5168566230efb
10 files changed