Enable -Wimplicit-fallthrough on release non-chromecast non-ozone linux builds.

clang will now warn on non-empty case and default blocks that don't end
with a break statement.  For cases where fallthrough is intentional,
end the block with FALLTHROUGH; from base/compiler-specific

Depends on these changes to DEPS rolling into chromium:
https://pdfium-review.googlesource.com/#/c/pdfium/+/24790
https://chromium-review.googlesource.com/c/angle/angle/+/895728
https://chromium-review.googlesource.com/c/v8/v8/+/895704
https://chromium-review.googlesource.com/c/breakpad/breakpad/+/895705
https://github.com/google/cld3/pull/9

We'll enable this for more build configurations once they can build
with the warning enabled.

(Also change a single file that was needed to get the dcheck_always_on
build to work, and another one that is built only in static builds.)

Bug: 177475,808047
Change-Id: Ic6a3fbf78a9cb2b5e928ce7973e67e4a39629dfd
Reviewed-on: https://chromium-review.googlesource.com/895726Reviewed-by: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533729}

build/config/compiler/BUILD.gn

@@ -8,6 +8,7 @@ import("//build/config/chromecast_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/coverage/coverage.gni")
 import("//build/config/host_byteorder.gni")
+import("//build/config/ui.gni")
 import("//build/toolchain/cc_wrapper.gni")
 import("//build/toolchain/toolchain.gni")
 import("//build_overrides/build.gni")
@@ -1425,13 +1426,9 @@ config("chromium_code") {
 
   # Warn on missing break statements at the end of switch cases.
   # For intentional fallthrough, use FALLTHROUGH; from base/compiler_specific.h
-  if (is_clang) {
-    # TODO(thakis): Enable this once the codebase is ready,
-    # https://crbug.com/177475
-    # cflags_cc = [ "-Wimplicit-fallthrough" ]
-    if (is_mac || is_ios) {
-      # cflags_objcc += [ "-Wimplicit-fallthrough" ]
-    }
+  # TODO(thakis): Enable for all platforms, https://crbug.com/177475
+  if (is_clang && is_linux && !is_debug && !is_chromecast && !use_ozone) {
+    cflags += [ "-Wimplicit-fallthrough" ]
   }
 
   configs = [ ":default_warnings" ]

third_party/WebKit/Source/core/dom/DocumentLifecycle.cpp

@@ -222,6 +222,7 @@ bool DocumentLifecycle::CanAdvanceTo(LifecycleState next_state) const {
       // Otherwise, we can continue onwards.
       if (next_state == kCompositingClean)
         return true;
+      FALLTHROUGH;  // TODO(thakis): Intended? https://crbug.com/808047
     case kCompositingClean:
       DCHECK(!RuntimeEnabledFeatures::SlimmingPaintV2Enabled());
       if (next_state == kInStyleRecalc)

tools/ipc_fuzzer/fuzzer/mutator.cc

@@ -29,8 +29,8 @@ void FuzzStringType(T* value, unsigned int frequency,
                     const T& literal1, const T& literal2) {
   if (RandEvent(frequency)) {
     switch (RandInRange(5)) {
-      case 4: (*value) = (*value) + (*value);   // FALLTHROUGH
-      case 3: (*value) = (*value) + (*value);   // FALLTHROUGH
+      case 4: (*value) = (*value) + (*value); FALLTHROUGH;
+      case 3: (*value) = (*value) + (*value); FALLTHROUGH;
       case 2: (*value) = (*value) + (*value); break;
       case 1: (*value) += literal1; break;
       case 0: (*value) = literal2; break;