Pass GYP DEPTH variable to isolate.

This should make it possible to get rid of the ugly
relative paths in the isolate files.

I'm including an example for base_unittests.isolate for 
demonstrative purposes only. I can remove it (or convert all
.isolate files in Chromium) before landing.

BUG=343106
TEST=Successful compile+test on Linux using:
ninja -C out/Release base_unittests_run xdisplaycheck
tools/swarming_client/isolate.py run -s out/Release/base_unittests.isolated

Review URL: https://codereview.chromium.org/322403003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@276671 0039d316-1c4b-4281-b951-d872f2087c98

build/android/pylib/gtest/setup.py

@@ -130,6 +130,7 @@ def _GenerateDepsDirUsingIsolate(suite_name, isolate_file_path=None):
       '--isolated', isolated_abs_path,
       '--outdir', constants.ISOLATE_DEPS_DIR,
 
+      '--path-variable', 'DEPTH', constants.DIR_SOURCE_ROOT,
       '--path-variable', 'PRODUCT_DIR', constants.GetOutDirectory(),
 
       '--config-variable', 'OS', 'android',

build/isolate.gypi

@@ -82,6 +82,7 @@
 
         # Path variables are used to replace file paths when loading a .isolate
         # file
+        '--path-variable', 'DEPTH', '<(DEPTH)',
         '--path-variable', 'PRODUCT_DIR', '<(PRODUCT_DIR) ',
 
         # Extra variables are replaced on the 'command' entry and on paths in