platform2: Update to build against libchrome-271506 by default

This CL also makes the following changes in order to build buffet with
libchrome-271506:

1. https://codereview.chromium.org/176693003 makes the uint8 pointer
   returned by MessageReader::PopArrayOfBytes() const to make it clearer
   that ownership remains with MessageReader. This CL makes the
   corresponding changes in buffet.

2. Use TrimWhitespaceASCII and TRIM_ALL from the base namespace.

BUG=chromium:375032
TEST=`USE='attestation buffet' FEATURES=test emerge-$BOARD platform2`
TEST=Trybot runs on paladin, release, and chromiumos-sdk builders.

Change-Id: I3301d7d85818dc38ffdc0611d175e0c007012ae4
Reviewed-on: https://chromium-review.googlesource.com/200715
Reviewed-by: Chris Masone <cmasone@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>

buffet/exported_property_set_unittest.cc

diff --git a/buffet/exported_property_set_unittest.cc b/buffet/exported_property_set_unittest.cc
index 7687371..ce8a3fe 100644
--- a/buffet/exported_property_set_unittest.cc
+++ b/buffet/exported_property_set_unittest.cc
@@ -446,7 +446,7 @@
       kTestInterface3, kPathListPropName);
   dbus::MessageReader reader(response.get());
   dbus::MessageReader variant_reader(nullptr);
-  uint8* buffer;
+  const uint8* buffer;
   size_t buffer_len;
   ASSERT_TRUE(reader.PopVariant(&variant_reader));
   // |buffer| remains under the control of the MessageReader.

buffet/string_utils.cc

diff --git a/buffet/string_utils.cc b/buffet/string_utils.cc
index e122355..55c7d3f 100644
--- a/buffet/string_utils.cc
+++ b/buffet/string_utils.cc
@@ -37,7 +37,7 @@
   if (trim_whitespaces) {
     std::for_each(tokens.begin(), tokens.end(),
                   [](std::string& str) {  // NOLINT(runtime/references)
-      TrimWhitespaceASCII(str, TRIM_ALL, &str); });
+      base::TrimWhitespaceASCII(str, base::TRIM_ALL, &str); });
   }
 
   return tokens;
@@ -60,8 +60,8 @@
   }
 
   if (trim_whitespaces) {
-    TrimWhitespaceASCII(pair.first, TRIM_ALL, &pair.first);
-    TrimWhitespaceASCII(pair.second, TRIM_ALL, &pair.second);
+    base::TrimWhitespaceASCII(pair.first, base::TRIM_ALL, &pair.first);
+    base::TrimWhitespaceASCII(pair.second, base::TRIM_ALL, &pair.second);
   }
 
   return pair;