[cleanup] initialize all struct fields

This allows to enable -Wmissing-field-initializers in a future CL.

Change-Id: I67ac828be97bf4f283e97486981adebaf8e4ebf9
Reviewed-on: https://chromium-review.googlesource.com/957731
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51904}

src/wasm/function-body-decoder.cc

@@ -46,7 +46,7 @@ struct SsaEnv {
     locals = nullptr;
     control = nullptr;
     effect = nullptr;
-    context_cache = {0};
+    context_cache = {};
   }
   void SetNotMerged() {
     if (state == kMerged) state = kReached;
@@ -750,7 +750,7 @@ class WasmGraphBuildingInterface {
     } else {
       result->state = SsaEnv::kUnreachable;
       result->locals = nullptr;
-      result->context_cache = {0};
+      result->context_cache = {};
     }
 
     return result;
@@ -778,7 +778,7 @@ class WasmGraphBuildingInterface {
     result->control = nullptr;
     result->effect = nullptr;
     result->locals = nullptr;
-    result->context_cache = {0};
+    result->context_cache = {};
     return result;
   }