clang/win: It's ok to pass /GS to clang-cl now.

We used to not pass /GS since clang-cl used to not implement buffer security
checks and would warn about this flag being ignored.  Now clang-cl implements
some form of buffer security checking and no longer warns on /GS, so that's
no longer necessary. This reverts the gyp bits of
https://codereview.chromium.org/1828543003/

/GS is also on by default, and as of https://codereview.chromium.org/1957523005
it's no longer explicitly passed in gn builds -- so no .gn files need to be
changed.

Since /GS is on by default, this CL doesn't change behavior.

BUG=598767

Review-Url: https://codereview.chromium.org/2129213002
Cr-Original-Commit-Position: refs/heads/master@{#404413}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: cc683aecb8e88d03139477b8eba81aeb1e3c2e9c

common.gypi

@@ -5703,6 +5703,7 @@
           'VCCLCompilerTool': {
             'AdditionalOptions': ['/MP'],
             'MinimalRebuild': 'false',
+            'BufferSecurityCheck': 'true',
             'EnableFunctionLevelLinking': 'true',
             'RuntimeTypeInfo': 'false',
             'WarningLevel': '4',
@@ -5775,14 +5776,6 @@
             }],
           ],
           'conditions': [
-            ['clang==0', {
-              'VCCLCompilerTool': {
-                 # TODO(thakis): Enable this with clang too,
-                 # https://crbug.com/598767
-                 'BufferSecurityCheck': 'true',
-              },
-            }],
-
             # Building with Clang on Windows is a work in progress and very
             # experimental. See crbug.com/82385.
             # Keep this in sync with the similar blocks in build/config/compiler/BUILD.gn