Support targets at the root level in generate_gradle.py.

Previously targets like //:foo wouldn't work because an extra slash would
be inserted in front of the GradleSubdir.

Bug: 620034
Change-Id: I93abfb36ccb683b7c6308a8cbe14e870ea699e00
Reviewed-on: https://chromium-review.googlesource.com/800617Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: Sami Kalliomäki <sakal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520585}

build/android/gradle/generate_gradle.py

@@ -191,7 +191,11 @@ class _ProjectEntry(object):
 
   def GradleSubdir(self):
     """Returns the output subdirectory."""
-    return self.NinjaTarget().replace(':', os.path.sep)
+    ninja_target = self.NinjaTarget()
+    # Support targets at the root level. e.g. //:foo
+    if ninja_target[0] == ':':
+      ninja_target = ninja_target[1:]
+    return ninja_target.replace(':', os.path.sep)
 
   def ProjectName(self):
     """Returns the Gradle project name."""