Support hermetic Xcode compilation with use_xcode_clang.

iOS builds can set use_xcode_clang = true and use_system_xcode = false when
chromium clang is broken.  This requires prepending DEVELOPER_DIR to clang/clang++, etc

BUG=669094

Review-Url: https://codereview.chromium.org/2578273002
Cr-Commit-Position: refs/heads/master@{#439028}

build/toolchain/mac/BUILD.gn

@@ -163,7 +163,7 @@ template("mac_toolchain") {
     tool("cc") {
       depfile = "{{output}}.d"
       precompiled_header_type = "gcc"
-      command = "$cc -MMD -MF $depfile {{defines}} {{include_dirs}} {{cflags}} {{cflags_c}} -c {{source}} -o {{output}}"
+      command = "$env_wrapper $cc -MMD -MF $depfile {{defines}} {{include_dirs}} {{cflags}} {{cflags_c}} -c {{source}} -o {{output}}"
       depsformat = "gcc"
       description = "CC {{output}}"
       outputs = [
@@ -174,7 +174,7 @@ template("mac_toolchain") {
     tool("cxx") {
       depfile = "{{output}}.d"
       precompiled_header_type = "gcc"
-      command = "$cxx -MMD -MF $depfile {{defines}} {{include_dirs}} {{cflags}} {{cflags_cc}} -c {{source}} -o {{output}}"
+      command = "$env_wrapper $cxx -MMD -MF $depfile {{defines}} {{include_dirs}} {{cflags}} {{cflags_cc}} -c {{source}} -o {{output}}"
       depsformat = "gcc"
       description = "CXX {{output}}"
       outputs = [
@@ -185,7 +185,7 @@ template("mac_toolchain") {
     tool("asm") {
       # For GCC we can just use the C compiler to compile assembly.
       depfile = "{{output}}.d"
-      command = "$cc -MMD -MF $depfile {{defines}} {{include_dirs}} {{asmflags}} -c {{source}} -o {{output}}"
+      command = "$env_wrapper $cc -MMD -MF $depfile {{defines}} {{include_dirs}} {{asmflags}} -c {{source}} -o {{output}}"
       depsformat = "gcc"
       description = "ASM {{output}}"
       outputs = [
@@ -196,7 +196,7 @@ template("mac_toolchain") {
     tool("objc") {
       depfile = "{{output}}.d"
       precompiled_header_type = "gcc"
-      command = "$cc -MMD -MF $depfile {{defines}} {{include_dirs}} {{cflags}} {{cflags_objc}} -c {{source}} -o {{output}}"
+      command = "$env_wrapper $cc -MMD -MF $depfile {{defines}} {{include_dirs}} {{cflags}} {{cflags_objc}} -c {{source}} -o {{output}}"
       depsformat = "gcc"
       description = "OBJC {{output}}"
       outputs = [
@@ -207,7 +207,7 @@ template("mac_toolchain") {
     tool("objcxx") {
       depfile = "{{output}}.d"
       precompiled_header_type = "gcc"
-      command = "$cxx -MMD -MF $depfile {{defines}} {{include_dirs}} {{cflags}} {{cflags_objcc}} -c {{source}} -o {{output}}"
+      command = "$env_wrapper $cxx -MMD -MF $depfile {{defines}} {{include_dirs}} {{cflags}} {{cflags_objcc}} -c {{source}} -o {{output}}"
       depsformat = "gcc"
       description = "OBJCXX {{output}}"
       outputs = [