Fix gclient_utils.FileRead() behavior for both 2 and 3.

We always want gclient_utils.FileRead() to return a text
string these days, but the existing code didn't work
right for Python2 if you called it explicitly with mode='r'.

R=ehmaldonado@chromium.org

Bug: 1210746
Change-Id: If540746f31308b671f9101334b5cd9848a7e9257
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2930626
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>

gclient_utils.py

@@ -171,14 +171,12 @@ def AskForData(message):
 
 
 def FileRead(filename, mode='rbU'):
-  # Always decodes output to a Unicode string.
-  # On Python 3 newlines are converted to '\n' by default and 'U' is deprecated.
-  if mode == 'rbU' and sys.version_info.major == 3:
-    mode = 'rb'
-  with open(filename, mode=mode) as f:
+  # mode is ignored now; we always return unicode strings.
+  with open(filename, mode='rb') as f:
     s = f.read()
-    if isinstance(s, bytes):
-      return s.decode('utf-8', 'replace')
+  try:
+    return s.decode('utf-8', 'replace')
+  except (UnicodeDecodeError, AttributeError):
     return s