Add BUILD.gn configuration.

This CL adds the necessary configuration to build glslang inside a
Chromium checkout. Two build warnings were fixed in the process to
make things compile.

SPIRV/SpvBuilder.cpp

@@ -631,7 +631,7 @@ Id Builder::getContainedTypeId(Id typeId) const
 // Returns true if 'typeId' is or contains a scalar type declared with 'typeOp'
 // of width 'width'. The 'width' is only consumed for int and float types.
 // Returns false otherwise.
-bool Builder::containsType(Id typeId, spv::Op typeOp, int width) const
+bool Builder::containsType(Id typeId, spv::Op typeOp, uint32_t width) const
 {
     const Instruction& instr = *module.getInstruction(typeId);