Enable tools/CrashHandler again.

  - Defaults to on the bots, triggered off CHROME_HEADLESS=1 in the environment.
  - Defaults off otherwise.
  - Can be enabled by setting CHROME_HEADLESS=1 (weird) or GYP_DEFINES=skia_crash_handler=1.

BUG=skia:
R=borenet@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/383813002
diff --git a/gyp/common_conditions.gypi b/gyp/common_conditions.gypi
index 0e4fca2..b1ce5fc 100644
--- a/gyp/common_conditions.gypi
+++ b/gyp/common_conditions.gypi
@@ -692,6 +692,10 @@
       ],
     }],
 
+    [ 'skia_crash_handler', {
+      'defines': [ 'SK_CRASH_HANDLER' ],
+    }],
+
   ], # end 'conditions'
   # The Xcode SYMROOT must be at the root. See build/common.gypi in chromium for more details
   'xcode_settings': {
diff --git a/gyp/common_variables.gypi b/gyp/common_variables.gypi
index 308c7d3..c3714dc 100644
--- a/gyp/common_variables.gypi
+++ b/gyp/common_variables.gypi
@@ -226,6 +226,8 @@
     'skia_run_pdfviewer_in_gm%': 0,
     'skia_disable_inlining%': 0,
     'skia_moz2d%': 0,
+    'skia_crash_handler%':
+        '<!(python -c "import os; print os.environ.get(\'CHROME_HEADLESS\', 0)")',
 
     # These are referenced by our .gypi files that list files (e.g. core.gypi)
     #