Cache condition evaluation results in gclient

Condition evaluation checks are fairly expensive, and turns out we
repeat a lot of checks. All those checks are also done in a single
thread.

With this cache on my workstation, a noop gclient runhooks takes 9.5s.
Without one, it takes 26.2s.

R=gavinmak@google.com

Change-Id: Ie00b7af22cd124d08dd9fd218de77b34bbdfe6ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3309823Reviewed-by: Gavin Mak <gavinmak@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>

gclient.py

@@ -643,6 +643,7 @@ class Dependency(gclient_utils.WorkItem, DependencySettings):
   def _deps_to_objects(self, deps, use_relative_paths):
     """Convert a deps dict to a dict of Dependency objects."""
     deps_to_add = []
+    cached_conditions = {}
     for name, dep_value in deps.items():
       should_process = self.should_process
       if dep_value is None:
@@ -651,9 +652,12 @@ class Dependency(gclient_utils.WorkItem, DependencySettings):
       condition = dep_value.get('condition')
       dep_type = dep_value.get('dep_type')
 
+
       if condition and not self._get_option('process_all_deps', False):
-        should_process = should_process and gclient_eval.EvaluateCondition(
-            condition, self.get_vars())
+        if condition not in cached_conditions:
+          cached_conditions[condition] = gclient_eval.EvaluateCondition(
+              condition, self.get_vars())
+        should_process = should_process and cached_conditions[condition]
 
       # The following option is only set by the 'revinfo' command.
       if self._get_option('ignore_dep_type', None) == dep_type: