Refactor OS_LINUX preprocessor directive for LaCrOS effort.

Currently, ChromeOS defines the OS_LINUX directive as well as
OS_CHROMEOS.  We're working to separate these two, so we're
making the fact that OS_LINUX == OS_LINUX || OS_CHROMEOS
explicit.

This is changes for /components/safe_browsing

This CL was uploaded by git cl split.

R=drubery@chromium.org

Bug: 1110266
Change-Id: Id895b4785eeea1828e2e2c753d24872412cf5984
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2370292Reviewed-by: Daniel Rubery <drubery@chromium.org>
Commit-Queue: Sean McAllister <smcallis@google.com>
Cr-Commit-Position: refs/heads/master@{#801275}

components/safe_browsing/core/db/v4_protocol_manager_util.cc

@@ -116,7 +116,7 @@ std::ostream& operator<<(std::ostream& os, const ListIdentifier& id) {
 PlatformType GetCurrentPlatformType() {
 #if defined(OS_WIN)
   return WINDOWS_PLATFORM;
-#elif defined(OS_LINUX)
+#elif defined(OS_LINUX) || defined(OS_CHROMEOS)
   return LINUX_PLATFORM;
 #elif defined(OS_IOS)
   return IOS_PLATFORM;