Do not use tool-wrapper for lld link

When building chrome on windows, tool-wrapper for linking is called 794 times.
And I found that calling python wrapper is relatively slow than calling link via ninja.

This CL replaces python wrapper to ninja wrapper to remove such overhead when we use lld.

I compared time of wrapper invocation using following powershell script.

for ($i = 0; $i -lt 100; $i++) {
    # ninja -t msvc -e environment.x64 -- C:/src/depot_tools/win_tools-2_7_6_bin/python/bin/python.exe -c ' '
    # python.exe ../../build/toolchain/win/tool_wrapper.py link-wrapper environment.x64 False C:/src/depot_tools/win_tools-2_7_6_bin/python/bin/python.exe -c ' ' # code for FlushFileBuffers is commented out.
}

* ninja wrapper, 7.451s for 100 invocation
* python wrapper, 17.803s for 100 invocation
Using ninja as env wrapper is more than 2 times faster.

Bug: 787903
Change-Id: I6531b59a6c43085782b0138a264ecfc9a6f65833
Reviewed-on: https://chromium-review.googlesource.com/1107678
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568836}

build/toolchain/win/BUILD.gn

@@ -133,6 +133,18 @@ template("msvc_toolchain") {
       sys_include_flags = ""
     }
 
+    if (use_lld) {
+      # Invoke ninja as wrapper instead of tool wrapper, because python
+      # invocation requires higher cpu usage compared to ninja invocation, and
+      # the python wrapper is only needed to work around link.exe problems.
+      # TODO(thakis): Remove wrapper once lld-link can merge manifests without
+      # relying on mt.exe being in %PATH% on Windows.
+      linker_wrapper = "ninja -t msvc -e $env -- "  # Note trailing space.
+    } else {
+      linker_wrapper =
+          "$python_path $tool_wrapper_path link-wrapper $env False "  # Note trailing space.
+    }
+
     clflags = ""
 
     # Pass /FC flag to the compiler if needed.
@@ -196,7 +208,8 @@ template("msvc_toolchain") {
 
     tool("alink") {
       rspfile = "{{output}}.rsp"
-      command = "$python_path $tool_wrapper_path link-wrapper $env False $lib /nologo {{arflags}} /OUT:{{output}} @$rspfile"
+      command =
+          "$linker_wrapper$lib /nologo {{arflags}} /OUT:{{output}} @$rspfile"
       description = "LIB {{output}}"
       outputs = [
         # Ignore {{output_extension}} and always use .lib, there's no reason to
@@ -218,7 +231,7 @@ template("msvc_toolchain") {
       rspfile = "${dllname}.rsp"
       pool = "//build/toolchain:link_pool($default_toolchain)"
 
-      command = "$python_path $tool_wrapper_path link-wrapper $env False $link /nologo /IMPLIB:$libname /DLL /OUT:$dllname /PDB:$pdbname @$rspfile"
+      command = "$linker_wrapper$link /nologo /IMPLIB:$libname /DLL /OUT:$dllname /PDB:$pdbname @$rspfile"
 
       default_output_extension = ".dll"
       default_output_dir = "{{root_out_dir}}"
@@ -251,7 +264,7 @@ template("msvc_toolchain") {
       rspfile = "${dllname}.rsp"
       pool = "//build/toolchain:link_pool($default_toolchain)"
 
-      command = "$python_path $tool_wrapper_path link-wrapper $env False $link /nologo /DLL /OUT:$dllname /PDB:$pdbname @$rspfile"
+      command = "$linker_wrapper$link /nologo /DLL /OUT:$dllname /PDB:$pdbname @$rspfile"
 
       default_output_extension = ".dll"
       default_output_dir = "{{root_out_dir}}"
@@ -275,7 +288,8 @@ template("msvc_toolchain") {
       rspfile = "$exename.rsp"
       pool = "//build/toolchain:link_pool($default_toolchain)"
 
-      command = "$python_path $tool_wrapper_path link-wrapper $env False $link /nologo /OUT:$exename /PDB:$pdbname @$rspfile"
+      command =
+          "$linker_wrapper$link /nologo /OUT:$exename /PDB:$pdbname @$rspfile"
 
       if (host_os == "win") {
         shellprefix = "cmd /c"