Add documentation about no sources for GN guide.

tools/gn/docs/style_guide.md

 # GN Style Guide 
 
 [TOC]
 ## Naming and ordering within the file
 
 ### Location of build files
 
 It usually makes sense to have more build files closer to the code than
 fewer ones at the toplevel (this is in contrast with what we did with
 GYP). This makes things easier to find and owners reviews easier since
@@ skipping 165 matching lines @@
     means the tests will get stripped.
 
   * Static libraries involve duplicating all of the data in the object files
     that comprise it. This takes more disk space and for certain very large
     libraries in configurations with very large object files can cause
     internal limits on the size of static libraries to be exceeded. Source
     sets do not have this limitation. Some targets switch between source sets
     and static libraries depending on the build configuration to avoid this
     problem.
 
+  * Source sets can have no sources, while static libraries will give strange
+    platform-specific errors if they have no sources. If a target has only
+    headers (for include checking purposes) or conditionally has no sources on
+    sone platforms, use a source set.
+
   * In cases where a lot of the symbols are not needed for a particular link
     (this especially happens when linking test binaries), putting that code in
     a static library can dramatically increase linking performance. This is
     because the object files not needed for the link are never considered in
     the first place, rather than forcing the linker to strip the unused code
     in a later pass when nothing references it.
 
 ### Loadable modules versus shared libraries versus components
 
 A component is a Chrome primitive (rather than a built-in GN concept) that
@@ skipping 74 matching lines @@
 `enable_google_now`, `enable_nacl`, `enable_remoting`, `enable_pdf`)
 
 `disable_foo` - _NOT_ recommended, use `enable_foo` instead with swapped default
 value
 
 `is_foo` - usually a global state descriptor (e.g. `is_chrome_branded`,
 `is_desktop_linux`); poor choice for non-globals
 
 `foo_use_bar` - prefixes can be used to indicate a limited scope for an argument
 (e.g. `rtc_use_h264`, `v8_use_snapshot`)