A starting point for doing all of src, and adding a PRESUBMIT.

Includes https://codereview.chromium.org/772663002/ and https://codereview.chromium.org/770053002/.
I haven't pushed new binaries yet.

Generated via:
> cd build
> git ls-files *.gn *.gni | sed -e "s/^/@..\\\\out\\\\Debug\\\\gn format --in-place /" >x.bat && x.bat

The only things that I don't love in the current output are:

1. Turning

    args = [
      "--depfile", rebase_path(depfile, root_build_dir),
      "--android-sdk-tools", rebased_android_sdk_build_tools,
      "--dex-path", rebased_output,
    ]

into:

    args = [
      "--depfile",
      rebase_path(depfile, root_build_dir),
      "--android-sdk-tools",
      rebased_android_sdk_build_tools,
      "--dex-path",
      rebased_output,
    ]

The heuristic for this isn't trivial though, and it also affects e.g. '-Xclang' in cflags, as well
as assignments to temporaries that are later assigned to args.

2. Turning single line

    if (defined(invoker.inputs)) { inputs = invoker.inputs }

into

    if (defined(invoker.inputs)) {
      inputs = invoker.inputs
    }

This could be argued to be an improvement, but as it's very boilerplate-y perhaps an exception to
allow single line in this case is worthwhile. I think there was discussion of new syntax for this
case too, something like "inputs ?= invoker.inputs" maybe.

In both cases, I think it's worthwhile to get formatting turned on, and then go back and special
case these if we decide it's worthwhile.

R=brettw@chromium.org
BUG=348474

Review URL: https://codereview.chromium.org/766573003

Cr-Commit-Position: refs/heads/master@{#306305}