gcapi: Bump -mmacosx-version-min= from 10.5 to 10.6

Clang and the macOS SDK are starting to drop support for targeting versions
before 10.6.

Bug: 874997
Change-Id: Id3ac6b42f49e0cdb28f85f127fb41305ba1af4cb
Reviewed-on: https://chromium-review.googlesource.com/1179748Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584031}

chrome/installer/gcapi_mac/BUILD.gn

@@ -10,12 +10,12 @@ config("gcapi_config") {
   cflags = [
     "-isysroot",
     sysroot,
-    "-mmacosx-version-min=10.5",
+    "-mmacosx-version-min=10.6",
   ]
   ldflags = [
     "-isysroot",
     sysroot,
-    "-mmacosx-version-min=10.5",
+    "-mmacosx-version-min=10.6",
   ]
 }