Re-land r405933 w/ fix for CrOS.

This CL re-lands r405933 and removes the hack that was needed while
rolling in the reworked v8_target_cpu code, but this adds the missing
line that was needed for CrOS to work correctly with their custom
toolchain.

TBR=machenbach@chromium.org, llozano@chromium.org
BUG=593461, 625353

Review-Url: https://codereview.chromium.org/2158143002
Cr-Commit-Position: refs/heads/master@{#406115}

build/toolchain/cros/BUILD.gn

@@ -127,6 +127,7 @@ gcc_toolchain("v8_snapshot") {
   } else {
     toolchain_cpu = "x64"
   }
+  v8_toolchain_cpu = v8_target_cpu
   toolchain_os = "linux"
   use_sysroot = false
 }

build/toolchain/gcc_toolchain.gni

@@ -475,12 +475,6 @@ template("gcc_toolchain") {
         v8_current_cpu = current_cpu
       }
 
-      # TODO(crbug.com/625353) - Delete after v8 has been updated
-      # to only refer to v8_current_cpu. Until then, we need to make
-      # sure that v8_current_cpu always has the same value as v8_target_cpu,
-      # so that //build defines evaluate the way v8 is expecting them to.
-      v8_current_cpu = v8_target_cpu
-
       # Disable sanitizers for non-default toolchains.
       is_asan = false
       is_cfi = false