Fix v8_context_snapshot_generator incremental builds

Add a v8_maybe_snapshot dependency to generate_v8_context_snapshot so
it is rebuilt when the v8 snapshot changes. Without this change, in
component builds v8 changes would cause libv8.so to rebuild but would
rarely trigger a v8 context snapshot build.

Fixed: 865389
Change-Id: Ib35b564bc28147fcb090c3b7f7fa852c11846684
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1878249Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709461}

tools/v8_context_snapshot/BUILD.gn

@@ -67,13 +67,24 @@ if (use_v8_context_snapshot) {
       "--output_file=$output_path",
     ]
 
-    outputs = [
-      output_file,
-    ]
-
     deps = [
       ":v8_context_snapshot_generator($v8_snapshot_toolchain)",
     ]
+
+    if (is_component_build) {
+      # In component builds, this forces the generator to rerun whenever the v8
+      # shared library is rebuilt. Without it would only regenerate if the
+      # generator itself was rebuilt.
+      inputs = [
+        "$root_out_dir/${shlib_prefix}v8${shlib_extension}",
+      ]
+
+      deps += [ "//v8" ]
+    }
+
+    outputs = [
+      output_file,
+    ]
   }
 
   # This config disables a link time optimization "ICF", which may merge different