Fix dependency for third_party/blink/renderer/bindings/modules/v8.

The v8 binding target has generated sources like
gen/third_party/blink/renderer/bindings/modules/v8/v8_media_recorder.cc
that then includes third_party/blink/renderer/modules/mediarecorder
headers. These headers use buildflags.h and all buildflags.h files are
generated. Thus for v8_media_recorder.cc to build, it needs to depend
on //third_party/blink/renderer/modules/mediarecorder:buildflags, but
that dependency is missing.

To fix this, add a dependency from
//third_party/blink/renderer/core:prerequisites, which is responsible
for generating all necessary C++ source code to
//third_party/blink/renderer/modules/mediarecorder:buildflags.

Bug: 1139543
Change-Id: If48a33cf4b5deb5bc66a8c21612eff924da774f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2482030
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820399}

third_party/blink/renderer/modules/BUILD.gn

@@ -256,6 +256,7 @@ group("make_modules_generated") {
     "//third_party/blink/renderer/bindings/modules:bindings_modules_generated",
     "//third_party/blink/renderer/bindings/modules/v8:generated",
     "//third_party/blink/renderer/core:core_event_interfaces",
+    "//third_party/blink/renderer/modules/mediarecorder:buildflags",
   ]
 }