Fix mb.py try command and docs

I seemed to have messed up and not uploaded the last changes I made
to https://crrev.com/c/1856822. I had meant to switch to ninja targets,
but the CL doesn't have those changes.

Bug: 1015682
Change-Id: Iefff69a47a682f2bc5ca549dc65348bd65536981
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1869836
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Auto-Submit: Stephen Martinis <martiniss@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707601}

tools/mb/docs/user_guide.md

@@ -171,7 +171,7 @@ to be run inside the isolate.
 Tries your change on the trybots. Right now this is essentially a fancy tryjob,
 like one you could trigger via `git cl try` or via CQ dry runs. Basic usage is
 
-`mb.py try -m tryserver.chromium.linux -b linux-rel //base:base_unittests`
+`mb.py try -m tryserver.chromium.linux -b linux-rel base_unittests`
 
 Your change must be uploaded to Gerrit. Local changes will not be uploaded for
 you. It uses the gerrit CL associated with your given git branch.
@@ -182,7 +182,7 @@ for a mapping of which bots are on which tryservers, and what those bots mirror.
 Any trybot in `trybots.py` is supported; you can test your code on windows, for
 example. The tryjob will compile and run your code on windows.
 
-The target (`base_unittests`) in the example is a GN build target. Most GN
+The target (`base_unittests`) in the example is a ninja build target. Most ninja
 unittest targets can be put here which currently runs on the bots.
 
 ### mb validate

tools/mb/mb.py

@@ -384,20 +384,11 @@ class MetaBuildWrapper(object):
     return 0
 
   def CmdTry(self):
-    target = self.args.target
-    if not target.startswith('//'):
-      self.Print("Expected a GN target like //:foo, got %s" % target)
+    ninja_target = self.args.target
+    if ninja_target.startswith('//'):
+      self.Print("Expected a nijna target like base_unittests, got %s" % target)
       return 1
 
-    recipe_name = None
-    isolate_map = self.ReadIsolateMap()
-    for name, config in isolate_map.iteritems():
-      if 'label' in config and config['label'] == target:
-        recipe_name = name
-        break
-    if not recipe_name:
-      self.Print("Unable to find a recipe entry for %s." % target)
-
     json_path = self.PathJoin(self.chromium_src_dir, 'out.json')
     try:
       ret, out, err = self.Run(
@@ -430,7 +421,7 @@ class MetaBuildWrapper(object):
       # TODO(martiniss): maybe don't always assume the bucket?
       'led', 'get-builder', 'luci.chromium.try:%s' % self.args.builder).then(
       'led', 'edit', '-r', 'chromium_trybot_experimental',
-        '-p', 'tests=["%s"]' % recipe_name).then(
+        '-p', 'tests=["%s"]' % ninja_target).then(
       'led', 'edit-cr-cl', issue_data['issue_url']).then(
       'led', 'launch').result