Update to build against libchrome-293518.

libchrome is updated from revision 271506 to 293518. This CL updates
platform2 code to build against libchrome-293518 with the following
changes:

- LOG_ERROR_REPORT is removed
  (https://codereview.chromium.org/331143007)

- StringToLowerASCII is moved to base namespace
  (https://codereview.chromium.org/448853002)

BUG=chromium:411001
CQ-DEPEND=CL:216584
CQ-DEPEND=CL:216585
CQ-DEPEND=CL:216586
CQ-DEPEND=CL:216511
CQ-DEPEND=CL:217084
CQ-DEPEND=CL:217085
TEST=Trybot run on paladin, release, and chromiumos-sdk builders.

Change-Id: I9fbdad30b3a7c79c1ec4e208664b8befea31a3ec
Reviewed-on: https://chromium-review.googlesource.com/216589
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>

metrics/libmetrics-293518.gyp

@@ -1,6 +1,6 @@
 {
   'variables': {
-    'libbase_ver': 271506,
+    'libbase_ver': 293518,
   },
   'includes': [
     '../metrics/libmetrics.gypi',

metrics/metrics.gyp

@@ -1,7 +1,4 @@
 {
-  'variables': {
-    'libbase_ver': 271506,
-  },
   'target_defaults': {
     'variables': {
       'deps': [