Implement target visibility in GN

Implement target visibility in GN

This implements a visibility property for determining which targets can depend on a given target.

The property works mostly like Blaze from the internal Google build, but replaces the magic public and private strings with wildcards, and defaults to public visibility.

R=scottmg@chromium.org, scottmg

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=260947

[brettw, 2014-03-28 21:06:47]

Unfortunately this includes another pattern parser and matcher. This one is
super simple at least. I was originally hoping to read a visibility string,
interpret it as a label, and run it through our existing pattern matching code.
But that didn't work because of the weird dual label/pattern nature of these
strings.

On the plus side, because the visibility patters are strict prefixes for
directories only, the matching is very fast.

[brettw, 2014-03-28 21:08:46]

See how this is used: https://codereview.chromium.org/217273004/

[scottmg, 2014-04-01 06:18:34]

lgtm, looks like it should useful to keep the browser/child DLLs separate on
Windows.

https://codereview.chromium.org/46313003/diff/190001/tools/gn/command_gen.cc
File tools/gn/command_gen.cc (right):

https://codereview.chromium.org/46313003/diff/190001/tools/gn/command_gen.cc#...
tools/gn/command_gen.cc:34: break;  // Don't return early since we need
DecrementWorkCount below.
Seems odd to RunAndWriteFile after an error?

https://codereview.chromium.org/46313003/diff/190001/tools/gn/visibility.cc
File tools/gn/visibility.cc (right):

https://codereview.chromium.org/46313003/diff/190001/tools/gn/visibility.cc#n...
tools/gn/visibility.cc:169: //   3. <non wildcard stuff>*  (send stuff throgh
dir resolution, note star)
"through"

[brettw, 2014-04-01 21:45:14]

Committed patchset #10 manually as r260947 (presubmit successful).