Add --force-compositing-mode to some virtual test suites.

Tools/Scripts/webkitpy/layout_tests/port/base.py

Index: Tools/Scripts/webkitpy/layout_tests/port/base.py
diff --git a/Tools/Scripts/webkitpy/layout_tests/port/base.py b/Tools/Scripts/webkitpy/layout_tests/port/base.py
index 094bb6d23bfd95be32967f99c08479b04d8201a4..4facfe5101a720d81cb85bac49e97f58f70199e0 100644
--- a/Tools/Scripts/webkitpy/layout_tests/port/base.py
+++ b/Tools/Scripts/webkitpy/layout_tests/port/base.py
@@ -1540,7 +1540,8 @@ class Port(object):
         return [
             VirtualTestSuite('gpu',
                              'fast/canvas',
-                             ['--enable-accelerated-2d-canvas']),
+                             ['--enable-accelerated-2d-canvas',
+                              '--force-compositing-mode']),
             VirtualTestSuite('gpu',
                              'canvas/philip',
                              ['--enable-accelerated-2d-canvas']),
@@ -1556,7 +1557,8 @@ class Port(object):
             VirtualTestSuite('softwarecompositing',
                              'compositing',
                              ['--disable-gpu',
-                              '--disable-gpu-compositing'],
+                              '--disable-gpu-compositing',
+                              '--force-compositing-mode'],
                              use_legacy_naming=True),
             VirtualTestSuite('deferred',
                              'fast/images',
@@ -1566,11 +1568,13 @@ class Port(object):
                              ['--enable-deferred-image-decoding', '--enable-per-tile-painting', '--force-compositing-mode']),
             VirtualTestSuite('gpu/compositedscrolling/overflow',
                              'compositing/overflow',
-                             ['--enable-accelerated-overflow-scroll'],
+                             ['--enable-accelerated-overflow-scroll',
+                              '--force-compositing-mode'],
                              use_legacy_naming=True),
             VirtualTestSuite('gpu/compositedscrolling/scrollbars',
                              'scrollbars',
-                             ['--enable-accelerated-overflow-scroll'],
+                             ['--enable-accelerated-overflow-scroll',
+                              '--force-compositing-mode'],
                              use_legacy_naming=True),
             VirtualTestSuite('threaded',
                              'animations',