Build v8_context_snapshot_generator with -pie on linux.

After https://chromium-review.googlesource.com/c/1293791,
Deterministic Linux should stay green with -pie, and it
makes that bot more like its dbg variant (.sos already
use position-independent code and ASLR) and like the
Windows deterministic bot.

Bug: 870584
Change-Id: I730437f3d1160016d3cdf78921990cf2d4b9a40a
Reviewed-on: https://chromium-review.googlesource.com/c/1294289Reviewed-by: Hans Wennborg <hans@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601660}

tools/v8_context_snapshot/BUILD.gn

@@ -110,5 +110,12 @@ if (use_v8_context_snapshot) {
       "//v8:external_startup_data",
       ":disable_icf",
     ]
+
+    if (is_linux) {
+      # Match main chrome binary, which has ASLR.
+      # Useful for making sure (via the Deterministic Linux bot)
+      # that no raw pointers get serialized.
+      ldflags = [ "-pie" ]
+    }
   }
 }