use_sysroot when possible.

Linux Chromium build uses headers and libraries in debian sysroot
instead of system's to provide hermetic build environment.

To compile simple chrome with Goma, builidng like this improve
cache hit ratio because we can expect compile happens in hermetic way.

Bug: 1003264
Change-Id: If7c5f80b5d732635f0b178efca0b9d4ca4a5c9e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1846427Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703625}

build/toolchain/cros/BUILD.gn

@@ -125,7 +125,7 @@ gcc_toolchain("host") {
     is_clang = cros_host_is_clang
     current_cpu = host_cpu
     current_os = "linux"
-    use_sysroot = false
+    use_sysroot = use_sysroot
   }
 }
 
@@ -168,6 +168,6 @@ gcc_toolchain("v8_snapshot") {
     }
     v8_current_cpu = v8_target_cpu
     current_os = "linux"
-    use_sysroot = false
+    use_sysroot = use_sysroot
   }
 }