Switch from -std=c++11 to -std=c++14.

The major blockers for this were that gcc+libc++ (android) and
clang+libstdc++4.8 (linux) didn't compile in this mode.  We've
since switched android from gcc to clang, and linux from libstdc++
to libc++, so this works fine everywhere now.

Off-waterfall bots setting `enable_custom_libcxx` to false need to
make sure that they use a libstdc++ newer than 4.8 (like the
simplechrome chromeos bots), else they'll get build errors -- see
the blocker bugs of the linked bug below.

We'll use the same process for C++14 that we used for C++11, we'll
list new features at http://chromium-cpp.appspot.com/, disallow
everything at first, and then allow things over time. So don't use
any C++14 stuff for now.

This doesn't affect the Windows bots; cl.exe (and clang-cl.exe)
have implicitly always allowed C++14.

Bug: 554717
Change-Id: Ibf2a19168a35b5aad2449af9abdc44a56997aeef
Reviewed-on: https://chromium-review.googlesource.com/583635Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490676}

build/config/compiler/BUILD.gn

@@ -459,14 +459,14 @@ config("compiler") {
     # TODO(thakis): Eventually switch this to c++11 instead,
     # http://crbug.com/427584
     cflags_c += [ "-std=gnu11" ]
-    cflags_cc += [ "-std=gnu++11" ]
+    cflags_cc += [ "-std=gnu++14" ]
   } else if (!is_win && !is_nacl) {
     # TODO(mcgrathr) - the NaCl GCC toolchain doesn't support either gnu11/gnu++11
     # or c11/c++11; we technically don't need this toolchain any more, but there
     # are still a few buildbots using it, so until those are turned off
     # we need the !is_nacl clause and the (is_nacl && is_clang) clause, above.
     cflags_c += [ "-std=c11" ]
-    cflags_cc += [ "-std=c++11" ]
+    cflags_cc += [ "-std=c++14" ]
   }
 
   if (is_mac) {