Add missing --debug flag to new layout test suites.

When I moved the test suites from chromium.webkit to the debug
builders on chromium.linux and chromium.mac in r576522, I
accidentally dropped the --debug flag; this causes the builds
to fail completely when it looks in the //out/Release directory
for the binaries.

BUG=853209
TBR=robliao@chromium.org
NOTRY=true

Change-Id: Ia36854052c07c3b94a1b42b128e769ed08fc160e
Reviewed-on: https://chromium-review.googlesource.com/1102657Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567676}

testing/buildbot/chromium.linux.json

@@ -2296,6 +2296,9 @@
         }
       },
       {
+        "args": [
+          "--debug"
+        ],
         "isolate_name": "webkit_layout_tests_exparchive",
         "merge": {
           "args": [

testing/buildbot/chromium.mac.json

@@ -2559,6 +2559,9 @@
         }
       },
       {
+        "args": [
+          "--debug"
+        ],
         "isolate_name": "webkit_layout_tests_exparchive",
         "merge": {
           "args": [

testing/buildbot/test_suite_exceptions.pyl

@@ -2530,7 +2530,7 @@
       'Linux Tests (dbg)(1)(32)', # 32-bit linux is unsupported
       # chromium.win
       'Win 7 Tests x64 (1)',  # 64-bit win is unsupported by the layout tests.
-      'Win10 Tests x64 (dbg)',  
+      'Win10 Tests x64 (dbg)',
     ],
     'modifications': {
       # chromium.fyi
@@ -2589,6 +2589,9 @@
         },
       },
       'Linux Tests (dbg)(1)': {
+        'args': [
+          '--debug',
+        ],
         'swarming': {
           'dimension_sets': [
             {
@@ -2654,6 +2657,11 @@
           'shards': 12,
         },
       },
+      'Mac10.13 Tests (dbg)': {
+        'args': [
+          '--debug',
+        ],
+      },
       # chromium.clang
       'ToTMac': {
         # TODO(thakis): Remove this once Mac10.13 Tests no longer runs tests on 10.12.