[fuchsia] Remove OS_FUCHSIA specific default discardable memory limit.

We now have a memory pressure signals implemented for Fuchsia, and even
when those are unavailable the lower 64MB discardable memory limit
can be set via --enable-low-end-device-mode=true.

Bug: 996030, 1020698
Change-Id: I24a76324f4fd8a6ec6c907a5437983c34d9f9908
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2056477
Auto-Submit: Wez <wez@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Peng Huang <penghuang@chromium.org>
Commit-Queue: Peng Huang <penghuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742582}

components/discardable_memory/service/discardable_shared_memory_manager.cc

@@ -163,12 +163,6 @@ int64_t GetDefaultMemoryLimit() {
   // Chromecast devices. Set value here as IsLowEndDevice() is used on some, but
   // not all Chromecast devices.
   int64_t max_default_memory_limit = 64 * kMegabyte;
-#elif defined(OS_FUCHSIA)
-  // Fuchsia doesn't implement MemoryPressureMonitor and the default limit is
-  // too high for some devices. Set it to the same value as for low-end devices.
-  // TODO(crbug.com/996030): Implement MemoryPressureMonitor for Fuchsia and
-  // remove this ifdef.
-  int64_t max_default_memory_limit = 64 * kMegabyte;
 #else
 #if defined(OS_ANDROID)
   // Limits the number of FDs used to 32, assuming a 4MB allocation size.