Fix typo in gn_isolate_map.pyl.

This fixes a typo introduced r419404, where the GN label for
webapk_shell_apk_junit_tests had one too many leading slashes.

TBR=brettw@chromium.org
NOTRY=true
BUG=555273

Review-Url: https://codereview.chromium.org/2349993002
Cr-Commit-Position: refs/heads/master@{#419409}

testing/buildbot/gn_isolate_map.pyl

@@ -909,7 +909,7 @@
     "type": "junit_test",
   },
   "webapk_shell_apk_junit_tests": {
-    "label": "///chrome/android/webapk/shell_apk:webapk_shell_apk_junit_tests",
+    "label": "//chrome/android/webapk/shell_apk:webapk_shell_apk_junit_tests",
     "type": "junit_test",
   },
   "webkit_unit_tests": {