GN: Use build directory for CD for scripts

This changes the current directory for script execution to be the build directory rather than the current BUILDfile directory. This makes things more consistent and simplifies some implementation.

It adds a new function to_build_path() for buildfiles to convert files to the build directory for this kind of call.

I removed all of the built-in relative* variables that were required to support this, and added one for target_out_dir and root_out_dir.

I added some tests and a test helper object since I was doing this kind of Scope setup a lot.

R=scottmg@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=223622

[scottmg, 2013-09-14 04:53:16]

ps3 didn't upload, but not much diff from ps2 i guess

lgtm

https://codereview.chromium.org/23606031/diff/3001/tools/gn/secondary/build/c...
File tools/gn/secondary/build/config/compiler/BUILD.gn (left):

https://codereview.chromium.org/23606031/diff/3001/tools/gn/secondary/build/c...
tools/gn/secondary/build/config/compiler/BUILD.gn:95:
"-B$relative_build_to_source_root_dir/third_party/gold",
... hmm, i see that's more like what you had before :)

https://codereview.chromium.org/23606031/diff/2/tools/gn/function_to_build_pa...
File tools/gn/function_to_build_path.cc (right):

https://codereview.chromium.org/23606031/diff/2/tools/gn/function_to_build_pa...
tools/gn/function_to_build_path.cc:67: const char kToBuildPath[] =
"to_build_path";
is this name from google3? i find it a bit confusing, mostly because of the
build dir vs. BUILD files i guess. 'relative_to_output' or something, not sure.

https://codereview.chromium.org/23606031/diff/2/tools/gn/function_to_build_pa...
tools/gn/function_to_build_path.cc:100: "    sources = [ \"foo.txt\",
\"bar.txt\" ]\n"
this ended up very confusing in gyp, and there's some ugly heuristics to decide
when to pathify lists of strings (include dirs, sources, bla), but some things
are "sort of" pathy like libraries. i don't have a suggestion, but worth
thinking about i guess.

[brettw, 2013-09-16 17:59:01]

https://codereview.chromium.org/23606031/diff/2/tools/gn/function_to_build_pa...
File tools/gn/function_to_build_path.cc (right):

https://codereview.chromium.org/23606031/diff/2/tools/gn/function_to_build_pa...
tools/gn/function_to_build_path.cc:67: const char kToBuildPath[] =
"to_build_path";
This is my own name. I agree the overlap of "build" is confusing and I'm open to
new names. I did "build" to not collide with the "out" dirs (like the
"root_out_dir" that I'm adding in this patch) which is the toolchain output
root, which is not necessarily the same as the root build directory. I could
call it "to_build_output_dir" but that seems unfortunately long.

I didn't really want to put "relative" in the name since this takes any file
name, often source-absolute ones like "//foo/bar.txt" or even system-absolute
ones like "/usr/include/foo.txt"

https://codereview.chromium.org/23606031/diff/2/tools/gn/function_to_build_pa...
tools/gn/function_to_build_path.cc:100: "    sources = [ \"foo.txt\",
\"bar.txt\" ]\n"
I think it's pretty clear here. Some things like sources and data are lists of
files. Args you pass manually are opaque and you have to manually convert them.

[brettw, 2013-09-17 16:54:28]

Committed patchset #5 manually as r223622 (presubmit successful).