Add inspector headers to top-level v8 target if inspector is enabled.

Otherwise, v8 embedders need to depend on targets in src.

BUG=chromium:688155

Change-Id: Idea90d8f550cd4d0141b92a7bdd18f4df664d11b
Reviewed-on: https://chromium-review.googlesource.com/447845
Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43631}

BUILD.gn

@@ -2268,6 +2268,10 @@ v8_source_set("v8_base") {
   }
 
   if (v8_enable_inspector) {
+    sources += [
+      "include/v8-inspector-protocol.h",
+      "include/v8-inspector.h",
+    ]
     deps += [ "src/inspector:inspector" ]
   }
 }