Update pdfium.py to support running pdfium tests with Dr. Memory

scripts/slave/recipes/pdfium.expected/drm_win_xfa.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]\\gclient.py",
       "config",
       "--spec",
       "cache_dir = None\nsolutions = [{'deps_file': 'DEPS', 'managed': True, 'name': 'pdfium', 'url': 'https://pdfium.googlesource.com/pdfium.git'}]"
     ],
@@ skipping 42 matching lines @@
     "cmd": [
       "ninja",
       "-C",
       "[SLAVE_BUILD]\\pdfium\\out\\Debug"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "compile with ninja"
   },
   {
     "cmd": [
-      "[SLAVE_BUILD]\\pdfium\\out\\Debug\\pdfium_unittests.exe"
+      "python",
+      "-u",
+      "[SLAVE_BUILD]\\pdfium\\tools\\drmemory\\scripts\\pdfium_tests.py"
+      "--test",
+      "pdfium_unittests"
     ],
     "cwd": "[SLAVE_BUILD]\\pdfium",
     "name": "unittests"
   },
   {
     "cmd": [
-      "[SLAVE_BUILD]\\pdfium\\out\\Debug\\pdfium_embeddertests.exe"
+      "python",
+      "-u",
+      "[SLAVE_BUILD]\\pdfium\\tools\\drmemory\\scripts\\pdfium_tests.py"
+      "--test",
+      "pdfium_embeddertests"
     ],
     "cwd": "[SLAVE_BUILD]\\pdfium",
     "name": "embeddertests"
   },
   {
     "cmd": [
       "python",
       "-u",
-      "[SLAVE_BUILD]\\pdfium\\testing\\tools\\run_javascript_tests.py"
+      "[SLAVE_BUILD]\\pdfium\\tools\\drmemory\\scripts\\pdfium_tests.py"
+      "--test",
+      "pdfium_javascript"
     ],
     "cwd": "[SLAVE_BUILD]\\pdfium",
     "name": "javascript tests"
   },
   {
     "cmd": [
       "python",
       "-u",
-      "[SLAVE_BUILD]\\pdfium\\testing\\tools\\run_pixel_tests.py"
+      "[SLAVE_BUILD]\\pdfium\\tools\\drmemory\\scripts\\pdfium_tests.py"
+      "--test",
+      "pdfium_pixel"
     ],
     "cwd": "[SLAVE_BUILD]\\pdfium",
     "name": "pixel tests"
   },
   {
     "cmd": [
       "python",
       "-u",
-      "[SLAVE_BUILD]\\pdfium\\testing\\tools\\run_corpus_tests.py"
+      "[SLAVE_BUILD]\\pdfium\\tools\\drmemory\\scripts\\pdfium_tests.py",
+      "--test",
+      "pdfium_corpus"
     ],
     "cwd": "[SLAVE_BUILD]\\pdfium",
     "name": "corpus tests"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]