Let clang-format sort includes.

After this, `git cl format` will reorder includes in blocks of #includes
that are not separated by newlines.

This works in almost all cases, but it can break some code e.g.

  #include <windows.h>
  #include <shellapi.h>

clang-format will reorder these now, but shellapi.h only compiles if
windows.h was included first. Relying on this is brittle, so replace
code like this with

  #include <windows.h>

  // Must be after windows.h:
  #include <shellapi.h>

Since clang-format doesn't reorder across blocks, this will do the right
thing.

This also means you're still on the hook of putting blocks with user headers,
C++ headers, and C headers in the right order.

This will hopefully replace src/tools/sort-headers.py which contains
some hacky heuristics -- but just inserting newlines between includes
when needed (with a comment) seems like a better tradeoff anyhow.
And the automatic integration with `git cl format` is nice.

(clang-format has IncludeIsMainRegex and IncludeCategories for adding
heuristics, but we shouldn't use these, they're too complicated.)

BUG=688155
TBR=brettw

Review-Url: https://codereview.chromium.org/2669263003
Cr-Commit-Position: refs/heads/master@{#447875}

.clang-format

@@ -6,6 +6,10 @@ BasedOnStyle: Chromium
 # 'vector<vector<int>>'. ('Auto' means that clang-format will only use
 # 'int>>' if the file already contains at least one such instance.)
 Standard: Cpp11
+
+# TODO(thakis): Default this to true in -style=Chromium if we decide to keep it.
+SortIncludes: true
+
 # Make sure code like:
 # IPC_BEGIN_MESSAGE_MAP()
 #   IPC_MESSAGE_HANDLER(WidgetHostViewHost_Update, OnUpdate)