Enable clang's -Wmissing-field-initializers warning.

This can protect against refactoring bugs when adding fields to an
aggregate-initialized struct.

Change-Id: Id2e9824a1adb8bf5dbdc3775dc59ee9f18c43412
Reviewed-on: https://chromium-review.googlesource.com/960324
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51907}

BUILD.gn

@@ -530,6 +530,8 @@ config("toolchain") {
 
   if (is_clang) {
     cflags += [
+      "-Wmissing-field-initializers",
+
       # TODO(hans): Remove once http://crbug.com/428099 is resolved.
       "-Winconsistent-missing-override",
     ]

test/cctest/test-assembler-mips.cc

@@ -10335,7 +10335,8 @@ TEST(MSA_fexdo) {
        -inf_float, 0, 0, 0, 0},
       {-0.f, 0.f, 123.567f, -765.321f, -6e-8f, 5.9e-8f, 1e-7f, -1e-20f, 0, 0, 0,
        0},
-      {1e-36f, 1e20f, -1e20f, 2e-20f, 6e-8f, -2.9e-8f, -66505.f, -65504.f}};
+      {1e-36f, 1e20f, -1e20f, 2e-20f, 6e-8f, -2.9e-8f, -66505.f, -65504.f, 0, 0,
+       0, 0}};
 
   const struct TestCaseMsa3RF_D tc_d[] = {
       // ws_lo, ws_hi, wt_lo, wt_hi, wd_lo, wd_hi

test/cctest/test-assembler-mips64.cc

@@ -11265,7 +11265,8 @@ TEST(MSA_fexdo) {
        -inf_float, 0, 0, 0, 0},
       {-0.f, 0.f, 123.567f, -765.321f, -6e-8f, 5.9e-8f, 1e-7f, -1e-20f, 0, 0, 0,
        0},
-      {1e-36f, 1e20f, -1e20f, 2e-20f, 6e-8f, -2.9e-8f, -66505.f, -65504.f}};
+      {1e-36f, 1e20f, -1e20f, 2e-20f, 6e-8f, -2.9e-8f, -66505.f, -65504.f, 0, 0,
+       0, 0}};
 
   const struct TestCaseMsa3RF_D tc_d[] = {
       // ws_lo, ws_hi, wt_lo, wt_hi, wd_lo, wd_hi