New GN binary pushed (possibly action required)

[Brett Wilson]

Last week I pushed r281094

ACTION REQUIRED FOR PEOPLE WITH STANDALONE BUILDS: The "deps" variable
inside the tool() part of a toolchain() definition is renamed
"depsformat" to avoid confusion with normal deps. Currently both are
supported. I'll fix the Chrome ones and them remove support for "deps"
in the next roll (probably 1-3 weeks). If you have a standalone build
with your own toolchain definitions, please s/deps/depsformat in that
time.

OTHER IMPORTANT CHANGE: "source_prereqs" is now renamed "inputs".
Currently both are supported. Please use "inputs" in all new code.
I'll do a search-and-replace pass through the main repo. We'll delete
"source_prereqs" support when everything has been updated.

Other changes:

- Deps for toolchains are now supported. This allows you to express
things that must be done before anything in that toolchain is
compiled, like compiling the compiler (or, in nacl's case, extracting
the compiler binary).

- Better messages for errors that occur in templates, now includes the "stack".

- Source expansions now have various relative directory options. (see
"gn help source_expansion"). For example, this lets you put output
files from action_foreach in "the gen directory corresponding go the
current input file" rather than just the current target gen dir. We
want this for protobufs.