Fix computation of relative paths in depsfile when cross-compiling

Fix computation of relative paths in depsfile when cross-compiling

Relative paths in a depsfile are interpreted by ninja relative to the
current working directory which in a GN build is the root of the build
directory ($root_build_dir in GN terms). When doing a
cross-compilation, this is different from the root of the output
directory ($root_out_dir in GN terms) for all builds other than the
target. This fixes the computation of depsfile entries so that they are
always relative to the current working directory.

Concretely, this means incremental builds in Fuchsia do not always
rerun the patch_sdk step for the host, shaving 12+ seconds off of no-op
builds.

R=johnmccutchan@google.com

Committed: https://github.com/dart-lang/sdk/commit/2b70abd2e3d52f0c6771f09b6847ae4bca3016e2

[jamesr1, 2017-03-27 21:41:54]

jamesr@google.com changed reviewers:
+ cutchccutchan@google.com, jamesr@google.com, vegorov@google.com,
zra@google.com

[jamesr1, 2017-03-27 21:41:54]

See discussion https://codereview.chromium.org/2729503006/

[zra, 2017-03-27 22:16:15]

zra@google.com changed reviewers:
+ johnmccutchan@google.com
- cutchccutchan@google.com

[Cutch, 2017-03-27 22:17:18]

lgtm

[jamesr, 2017-03-28 03:28:12]

Description was changed from

==========
Fix computation of relative paths in depsfile when cross-compiling

Relative paths in a depsfile are interpreted by ninja relative to the
current working directory which in a GN build is the root of the build
directory ($root_build_dir in GN terms). When doing a
cross-compilation, this is different from the root of the output
directory ($root_out_dir in GN terms) for all builds other than the
target. This fixes the computation of depsfile entries so that they are
always relative to the current working directory.

Concretely, this means incremental builds in Fuchsia do not always
rerun the patch_sdk step for the host, shaving 12+ seconds off of no-op
builds.
==========

to

==========
Fix computation of relative paths in depsfile when cross-compiling

Relative paths in a depsfile are interpreted by ninja relative to the
current working directory which in a GN build is the root of the build
directory ($root_build_dir in GN terms). When doing a
cross-compilation, this is different from the root of the output
directory ($root_out_dir in GN terms) for all builds other than the
target. This fixes the computation of depsfile entries so that they are
always relative to the current working directory.

Concretely, this means incremental builds in Fuchsia do not always
rerun the patch_sdk step for the host, shaving 12+ seconds off of no-op
builds.

R=johnmccutchan@google.com

Committed:
https://github.com/dart-lang/sdk/commit/2b70abd2e3d52f0c6771f09b6847ae4bca3016e2
==========

[jamesr, 2017-03-28 03:28:13]

Committed patchset #1 (id:1) manually as
2b70abd2e3d52f0c6771f09b6847ae4bca3016e2 (presubmit successful).

[ahe, 2017-03-28 06:23:50]

ahe@google.com changed reviewers:
+ ahe@google.com

[ahe, 2017-03-28 06:23:51]

lgtm

Thank you!