Import build/config/clang/clang.gni in webrtc/base/BUILD.gn

Currently,
https://build.chromium.org/p/chromium.fyi/builders/CrWinAsan%28dll%29/builds/...
is broken because of

Writing """\
clang_use_chrome_plugins = false
is_asan = true
is_clang = true
is_component_build = true
is_debug = false
llvm_force_head_revision = true
symbol_level = 2
target_cpu = "x86"
v8_enable_verify_heap = true
""" to C:\b\c\b\CrWinAsan_dll_\src\out\Release\args.gn.

C:\b\c\b\CrWinAsan_dll_\src\buildtools\win\gn.exe gen //out/Release --check
--runtime-deps-list-file=C:\b\c\b\CrWinAsan_dll_\src\out\Release\runtime_deps
  -> returned 1
ERROR at //third_party/webrtc/base/BUILD.gn:276:21: Undefined identifier in string expansion.
        data += [ "$clang_base_path/lib/clang/$clang_version/lib/windows/clang_rt.asan_dynamic-i386.dll" ]
                    ^--------------
"clang_base_path" is not currently in scope.
See //content/test/BUILD.gn:307:7: which caused the file to be included.
      "//third_party/webrtc/base:rtc_base",
      ^-----------------

NOTRY=True
NOPRESUBMIT=True
R=kjellander@webrtc.org, thakis@chromium.org
BUG=chromium:497757
TBR=kjellander

Review-Url: https://codereview.webrtc.org/2422223002
Cr-Commit-Position: refs/heads/master@{#14653}

webrtc/base/BUILD.gn

@@ -16,6 +16,7 @@ if (is_android) {
 }
 if (is_win) {
   import("//build/config/win/visual_studio_version.gni")
+  import("//build/config/clang/clang.gni")
 }
 
 config("rtc_base_approved_all_dependent_config") {