[build] Do not use absolute paths in v8_enable_vtunejit-related blocks.

This prevents the flag from being set from e.g. Chromium. Instead, just use
relative paths like everything else in the build system.

Bug: chromium:825347, v8:7601

Change-Id: I080d9999b0b63bafc2c1978f70322eb48814a3b8
Reviewed-on: https://chromium-review.googlesource.com/980557
Commit-Queue: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52263}

BUILD.gn

@@ -3163,7 +3163,7 @@ v8_executable("d8") {
   defines = []
 
   if (v8_enable_vtunejit) {
-    deps += [ "//src/third_party/vtune:v8_vtune" ]
+    deps += [ "src/third_party/vtune:v8_vtune" ]
   }
 }
 

src/third_party/vtune/BUILD.gn

@@ -15,6 +15,6 @@ static_library("v8_vtune") {
     "vtune-jit.h",
   ]
   deps = [
-    "//:v8",
+    "../../..:v8",
   ]
 }