[cpplint] add stdin support in stdin

Cpplint can already process input from stdin (when the filename is '-'),
but `git cl lint` currently lacks this capability.

Adding stdin support to `git cl lint` would enable seamless integration
with IDEs like Visual Studio. Linters could then operate directly on
unsaved code within the editor.

Bug: 372288610
Change-Id: Ib2d5a3534ac0c5afcfcac64b708becde41612881
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5917767
Commit-Queue: Scott Lee <ddoman@chromium.org>
Reviewed-by: Gavin Mak <gavinmak@google.com>

cpplint.py

@@ -6399,10 +6399,9 @@ def ProcessFile(filename, vlevel, extra_check_functions=[]):
         # If after the split a trailing '\r' is present, it is removed
         # below.
         if filename == '-':
-            lines = codecs.StreamReaderWriter(sys.stdin,
-                                              codecs.getreader('utf8'),
-                                              codecs.getwriter('utf8'),
-                                              'replace').read().split('\n')
+            lines = codecs.StreamReaderWriter(
+                sys.stdin.read().encode('utf8'), codecs.getreader('utf8'),
+                codecs.getwriter('utf8'), 'replace').decode('utf8').split('\n')
         else:
             with codecs.open(filename, 'r', 'utf8', 'replace') as stream:
                 lines = stream.read().split('\n')